Last October, detectives were called to investigate the death of a woman under a London tube train. But as they traced her final moments, they discovered that she was, in fact, David Burgess, one of the most brilliant immigration lawyers of his generation. Here, Burgess's family and friends tell, for the first time, the complicated story of the loving father, brilliant colleague, sensitive woman and courageous person they knew

On an autumn evening last October, a slight, pretty woman with a mass of curly hair fell underneath a tube train during rush hour at King's Cross underground station. The driver of the eastbound Piccadilly Line train applied the brakes as soon as he saw the woman lose her balance, but a whole carriage passed along the platform before the vehicle shuddered to a halt. It was shortly after 6.30pm on 25 October when the British Transport Police started trying to recover the body, a gruesome task that lasted late into the night.

The line was closed, the platform cleared. London's Underground network was severely disrupted as commuters struggled to make their way home. And yet, in the sprawling urban mass of the capital, many of those passengers – crushed against each other in scarves and coats, clutching their copies of the Evening Standard and adjusting their iPods – probably reflected that, depressing though it might be, a person throwing themselves in front of a tube train was not particularly out of the ordinary.

But all was not as it seemed. The ensuing media coverage revealed that the police suspected that the woman had not fallen but had been pushed by her 34-year-old female companion, who was later charged with murder. It then turned out that the woman who died, 63-year-old Sonia Burgess, was living a double life. Once the police had established her identity (from her railcard), it was discovered that Sonia was biologically a man – a man named David Burgess, one of the finest immigration lawyers of his generation, a man responsible for a succession of trailblazing judgments in the House of Lords and the European Court of Human Rights.

The confusion over the dead person's identity meant that it took longer than usual for the dreadful news to trickle through to friends and family. Christina Beardsley, who was friends with Sonia through a Christian transgender support and discussion group, read the evening newspaper's report of the death, but initially she did not think the victim was anyone she knew.

"Then a friend rang to tell me it was Sonia," she says. "I couldn't believe it. You never think it would be somebody you know, or even that they might have been pushed. It's a horribly violent way to die, isn't it? Living and working in London, you hear of it all the time – those delays because somebody is on the line – and often you think, 'Oh, why did they do that? What was going on?'" She pauses to fiddle with the handle of her latte cup, letting the thought dissolve into the air like a pricked bubble. Many of Sonia's friends found the media interest difficult to stomach, especially because some newspapers used the male pronoun to refer to Burgess in spite of the fact that he had chosen to live as a woman.

"Things like that can be very hurtful in the transgender community," says Beardsley. "It was only when I read an obituary that I learned about Sonia's professional history. She had this diffidence about her, an empathy for other people. She never said to me, 'By the way, do you know I've been dealing with all these high-profile legal cases?' She just said she was a human-rights lawyer. I just knew her as someone who was fun to be around: loving, sensitive, aware, in the present moment… a deep thinker, and, of course," Beardsley adds with a chuckle, "very fashionable. She had a very good eye for clothes. When I met her, she was almost apologetic that she still worked as a male."

But it was as a man that this self-effacing individual had, indeed, made legal history. As Beardsley was shortly to discover, Burgess had graduated in law from St Catharine's College, Cambridge in 1969 and gone on to co-found his own legal aid law firm, Winstanley Burgess solicitors, six years later. As senior partner, he began to specialise in asylum work.

In 1987, Burgess acted on behalf of a group of 52 Sri Lankan Tamil asylum seekers who were refused entry at the UK border and threatened with deportation. In an article written after his death, Frances Webber, then a junior barrister, remembers being asked by Burgess "to run across to the High Court to get an injunction to prevent [their] removal… When I say 'run', I mean it literally – immigration officials were escorting the Tamils to the plane, and they, hearing of David's efforts, decided to help by stripping off on the tarmac. We got our injunction – but eventually the men's judicial review claims were rejected and they were sent back to Sri Lanka. David didn't give up."

At the time, refugees had no right to appeal against deportation before being sent back to their native country, but Burgess refused to take no for an answer. He travelled to Sri Lanka at the firm's expense to gather evidence of the men's maltreatment, eventually winning them the right to return to the UK. The law was changed as a result of the case, to ensure asylum seekers could appeal against refusal of asylum before being sent home.

The same dogged determination was in evidence when Burgess took up the cause of "M", a Zairean asylum seeker, in 1991. He eventually brought contempt proceedings against Kenneth Baker, then the Conservative home secretary, for failing to stop M's deportation. The House of Lords upheld Burgess's complaint, in a decision which reversed centuries of deference to Crown ministers. The victory was described by Professor Sir William Wade, one of the giants of academic law, as the most significant constitutional case for more than 200 years.

In a tribute composed shortly after Burgess died, his former colleague Chris Randall recalled his former mentor's "unique imagination" in dealing with complex legal issues. "He inspired great client loyalty," Randall said. In fact, two of the Tamil asylum seekers whom Burgess had helped in 1987 visited him every year for over a decade after their case was finished. "Not surprisingly," Randall writes, "a number of his clients were inspired themselves to become lawyers."

Burgess's dedication to his work was absolute: he displayed an extraordinary level of compassion for his clients, often finding them places to stay or lending them money while they fought their legal battles. In 1991, when a Tibetan official who was visiting London on a Chinese government delegation decided to defect, Burgess put him up in his own home for several weeks before the man was granted asylum.

Five years later, Burgess acted for an alleged Sikh militant, Karamjit Singh Chahal, who was facing deportation to India – where, he claimed, he would be at risk of torture. The European Court of Human Rights ruled against deportation, stating that protection against the risk of torture should be absolute even if those at risk posed a security threat to the UK – a judgment instrumental in preventing the deportation of terrorist suspects after 9/11.

Robert Winstanley, a Cambridge contemporary who co-founded Winstanley Burgess, and who is now a judge, recalls that David "always worked enormous hours. His client care was over and above what was necessary to conduct their case. He was very pastoral with his clients, much more than I ever was. If they needed a kindly word at 2am, they would get it from David."

Alongside his work with refugees and asylum seekers, Burgess represented transgender clients in a series of ground-breaking cases, in which he sought to win full legal recognition for his clients' changed gender status. In the 1980s, he acted on behalf of Mark Rees, a female-to-male transsexual who wished to have the gender on his birth certificate altered, and he also campaigned for the right of Stephen Whittle, a female-to-male transsexual parent to be legally recognised as the father of his children. Whittle, who wrote an online obituary referring to Burgess in female terms, remembers his lawyer coming up with "brilliant strategies… She would say they came to her in the shower." Burgess, he continues, had "a mission… to rescue the earth's dispossessed."

It was a mission that would make David Burgess one of the finest lawyers of his generation. Jawaid Luqmani, who knew and worked with him for 20 years, says: "David was somebody everybody I knew wanted to be. He was extraordinarily talented, with an encyclopedic grasp of the law; we all wanted to be as good as he was. But he had almost no ego – which, as a lawyer, is practically unheard of."

Professionally, it seemed Burgess was drawn to defending the marginalised: the misfits and the outsiders who were too readily ignored or persecuted by the mainstream. And it is clear that, in his personal life, he underwent his own struggles with identity and social acceptance. Chris Randall puts it thus: "In many ways, he was something of an outsider himself; his was a position of empathy with the client, not sympathy. He did not do 'professional detachment'… We can speculate as to whether he enjoyed the way that he compartmentalised elements of his life or whether that was the only defence he had against a world which does not cope well with individuality such as his."

Was he better able to relate to an asylum seeker's plight because of his own awareness of how difficult it could be to fit in, to find one's place in the world? Perhaps. But whereas Burgess was able to give so many of his clients the peace of mind they longed for, there is, for many of his friends, a sadness in the realisation that he seemed unable to achieve a similar resolution in his own life.

On the day that Burgess was killed, he was living as a woman and yet working as a man. He was open about his lifestyle to anyone who asked, but he also had separate groups of friends: those who knew him as David and those who knew him exclusively as Sonia. He allowed only his closest confidants to see him in both guises and, as numerous of his acquaintances will attest, he shied away from mentioning his achievements, preferring instead to draw out the person to whom he was talking. The vast majority of those who knew him as Sonia, such as Christina Beardsley, knew nothing of his remarkable professional record. Those who knew him as David had little inkling that, for the past five years, he had been living as a vivacious and attractive woman, shopping for clothes in Zara, getting his ears pierced and undergoing electrolysis to remove his facial hair.

When Sonia fell in front of that tube train last October, the rush-hour commuters could have had no idea about the extent of her uniqueness. And for many of Sonia's friends, her untimely death was the final tug on a ball of thread that would unravel her extraordinary story.

When Ian Baker first met David Burgess, he was struck by his macho character. It was 1966 and the two of them were law undergraduates at St Catharine's College, Cambridge. "I remember he'd done all sorts of Duke of Edinburgh Awards and used to talk about doing athletic things," says Baker, now a district judge, sitting in a low armchair in his small, strip-lit office in Highbury Corner Magistrates' Court in London. He recalls Burgess once climbing under a bridge on the River Cam, using the metal latticework to clamber across by hand. "Of course, he slipped and went straight in the Cam," says Baker, laughing. "But he always gave that impression of being quite physical, quite macho, which was rather different from the character which developed."

The two of them became close friends, drawn together by a shared background. "David came from Castleford, in West Yorkshire, a mining community. I came from a village in South Wales on the edge of a mining community. We both had working-class backgrounds and we both felt different from the other eight [law students] in our year, all of whom were public schoolboys. We bonded straightaway and became very close friends during that first year."

Both, too, had been raised by single mothers: Burgess's father had left the family home when Burgess was still very young. He and his older sister, Ros, were brought up by their mother, Comfort, a secondary school headmistress who was, by all accounts, a formidable woman – prim, small and slight, but a stickler for discipline.

Burgess never mentioned his father, although I am told he was aware of his existence and there is some speculation among his friends that his father could have been mentally unstable and incarcerated in an asylum. Whatever the reason, Baker suspects that the lack of a male parent "was the source of some trouble for David. I'm sure it was a significant element for him… there was always a side to David that was not at peace. It's difficult to put your finger on what it was. I think 'outsider' is an appropriate description. He felt something of a fish out of water at Cambridge."

At university, Burgess started to experiment with his sexuality. He had a long-term girlfriend from Hartlepool who found work as a secretary in Cambridge in order to be with him, but at the same time, he began to cultivate friendships with undergraduates who were openly homosexual. "David's sense of sexual identity was always something of an issue for him," says Baker. "He spent quite a lot of time with an effete crowd at Cambridge, but I was not aware of his being homosexual at that time."

It was not until the mid-1970s, by which time Burgess had set up his own legal aid firm with Robert Winstanley, in rooms above a Pizza Hut in Islington, north London, that his friends became aware of his transvestism. Around this time he asked Baker, then a married father of one, to take him out for an evening as a female. "That was the first idea I had that David had any interest in dressing as a woman," he recalls. "I did it, even though it was completely outside my sphere of interest, because he was a friend and it mattered to him. We went out in Covent Garden. I found it awkward and very uncomfortable because he was not at all a convincing woman at that time. But it was clearly something he needed and wanted to do."

For a while, Burgess pursued a gay relationship with a young Chinese man called Sonny, who lived in his flat in Bedford Square, Bloomsbury. He was open about his sexuality – at one point, he even took Sonny up to visit his family in Castleford and insisted on going to a rugby league match with him. "That takes balls," says Baker, with a quiet laugh. "He had courage."

It was this courage that endeared him to so many people. What strikes me most through the course of my conversations with those who loved and knew David Burgess is their complete acceptance of his complex sexuality, even though the choices he made were often outside the sphere of their own experience or understanding. Again and again, Burgess's diverse group of friends talk of his incredible warmth, his generosity of spirit, his kindness. The words that recur most frequently are "diffidence", "self-effacement" and "humility" – the sense that, in spite of all he would go on to achieve, Burgess remained uncomfortable in the spotlight, preferring instead to concentrate on helping those who needed it most. "If you met him, he was a very gently spoken person," says Robert Winstanley. "But there was iron in the soul."

Burgess was drawn to vulnerability, perhaps because of his own fragile sense of self. But he also possessed an astonishing drive – that "iron in the soul" – to protect those less fortunate; to stand up for what he believed was right. At Winstanley Burgess, he immersed himself in immigration law, rapidly building up a considerable reputation as the firm expanded.

Through his work, he met Youdon Lhamo, a female Tibetan refugee who was working as a nurse, and in 1985 the couple married. Burgess told Youdon about his sexual past and his interest in transvestism. According to close friends, the marriage was not without its challenges, but Youdon and David were to remain married for 20 years. (Although she was approached for the purposes of this article, Youdon did not wish to comment.) They had a son, Tenzin, a daughter, Kusang, and in 1987 they adopted Dechen, Youdon's seven-year-old niece, whose family had been living in exile in northern India. (The children now live abroad – Kusang and Dechen in Canada, and Tenzin in the Czech Republic, where he is an English teacher.) As soon as the children were old enough they, too, were told of their father's female alter-ego, and seemingly took it in their stride. When Tenzin was informed, he replied, jokingly: "Dad, at last something interesting about you!"

Sonia and daughter Kusang after a night out, taken by adopted daughter Dechen. Photograph: Image courtesy of family

For the children, there was never any sense that their father was conflicted or tortured: both David and Sonia were, in their eyes, the same loving parent who would leave print-outs of homework topics outside their bedroom doors and who would drop everything to meet them for coffee if they happened to be passing his office.

In many ways, their father's shifting identity added an extra dimension to their relationship: as she grew older, Dechen would talk about make-up and clothes with Sonia. All three of them would visit art galleries or museums with her. A close friend of the family recalls Burgess leading by example: "Sonia taught the children the importance of love and tolerance and being courageous about living true to oneself. She was encouraging, supportive, kind, with a great sense of humour and, perhaps most importantly, the children knew how much she loved them and were always secure in the knowledge that her love was unconditional."

Ian Baker agrees. Burgess's sexuality, he says, "was never a problem or an issue with his family. I do know that David was a workaholic, and that's always a difficult thing with children."

In fact, Burgess's workload intensified through the 1990s. He would put in 13-hour days, popping back to the family home in Corsica Street, in Highbury, for supper and then returning to the office to work late into the night. Often, he would be dealing on a one-to-one basis with clients in extreme situations, traumatised by torture, rape and civil war. Apart from the draining personal impact of such work, the bureaucratic form-filling (never one of Burgess's strong points) was immensely time-consuming, especially after the Legal Services Commission introduced a series of reforms designed to halt the spiralling costs of the legal aid budget.

In 1996, Robert Winstanley left the firm to become a judge. The departure was amicable and the two of them continued to meet regularly for games of snooker followed by bangers and mash at the RAC Club in Pall Mall. But in 2003, Burgess, by then burned out from years of intense commitment, allowed the firm to fold. He spent a year in Tibet, studying Lamaism (Tibetan Buddhism) and learning the language. On his return in 2005 he separated from his wife: Baker remembers meeting him at this time and finding him "at a very low ebb". Burgess had lost both his marriage and the firm he had established. The work he felt so passionately about had exhausted him, mentally and physically. His identity began to fragment and, at some point around this time, issues around his gender seemed to become more pronounced. He began working part-time at the immigration law firm Luqmani Thompson in north London and for the Medical Foundation for the Care of Victims of Torture. In his professional life, Burgess remained a man. In his private life, however, he started to live as Sonia full-time.

Tara, a transgender woman, met Sonia Burgess five years ago, when she came to her flat for a party. Sonia – slim, elegant, with long legs and a ready smile – was one of half a dozen people who stayed behind afterwards to help clear up. "I knew the day I met her that this was someone there was a lot to," says Tara (not her real name). "She had a delightfully oblique way of looking at things, a real intelligence, a sense of humour and warmth."

As the two of them became closer, Sonia revealed a minimal amount about her day job, saying that she was a lawyer with an interest in immigration. "She wasn't given to boasting," says Tara. "I didn't realise the extent of her achievements until I read the obituaries." Although Sonia was open about the fact that she still went to work as a man, Tara saw her in male clothes only once – when Tara was helping her find a flat and Sonia came straight from work to view a property during her lunch break.

The flat, situated in Cambridge Circus in central London, would become Sonia's new home. The cobbled streets and bright lights of Soho were just across the road and she came to feel comfortable in this part of town, even joining the local church, St Anne's, which was renowned for its tolerant and inclusive approach.

Sonia's increasing confidence was reflected in her physical appearance. Always slender as David, Sonia became even thinner. According to Tara: "I think gender dysmorphia gets stronger as you get older. You can fight it when you're young but in the end, you think 'I'm sick of this, I can't do this any more. This is not how I want to live my life.' I think that's what happened with Sonia. She was growing her hair out and she was certainly discussing some facial surgery and a boob job, but I don't think she was going to go for genital surgery. I don't think she thought it was worth it. It was enough for her to be perceived as female. She didn't see the need to go through such a radical procedure."

Partly, this was because Burgess seemed to feel little friction between his two gender identities. "It was about somebody being themselves but in different modes," says Christina Beardsley. "I think Sonia was happy. I didn't see any tension there." Burgess's family remained remarkably supportive of their father's transition – Sonia's eldest daughter, Dechen, a trained beautician, would sometimes paint Sonia's nails. Burgess also kept in touch with his Castleford roots – his mother died in the 1990s but he was in regular contact with his sister, Ros, who was diagnosed with Parkinson's disease a few years ago, and whom he called on an almost daily basis. Ian Baker first encountered Sonia when he was invited to a memorable party at the Cambridge Circus flat more than a year ago, when the door was opened by a vivacious, attractive woman who went to great lengths to set all her guests at ease. "Seeing David then, I thought he'd finally reached something that was a resolution of what had obsessed or troubled him right from the beginning," says Baker. "I felt really happy to see him. He seemed so happy…" He breaks off, his voice choking. "One can get poetic about these things, but I think he was in his chrysalis stage for a long time and I think Sonia was the butterfly that emerged."

For a brief period of time after he began living as Sonia, Burgess advertised herself as a pre-operative transsexual escort on specialist websites, going by the name of Sonia Jardiniere and looking for paid encounters with men. Much was made of this fact in the aftermath of his death and the sensationalised tone of some of the media coverage has caused great distress to Burgess's friends and family, and made them understandably wary of the press – indeed, this is the first time any of them have spoken openly.

"She dabbled with being an escort, as many of us do," explains Tara, "because it was fun, and also it's a kind of validation. If you're trying to be female, having a man fancy you is the ultimate compliment, isn't it? But she actually had no interest in casual sex. She was worried about old age, about being alone. She wanted love." And for a while before she died, Sonia did find love, but it was a love that came with its own challenges: her boyfriend of the past three years was a married man who could never be wholly open about their relationship.

At Luqmani Thompson, the law firm where Burgess still worked as David, the existence of his alter ego was something of an open secret. "He never came to work as Sonia but that, I think, was more to do with David wanting to make sure everyone else was OK," says Jawaid Luqmani, the firm's founding partner. "That would be his greatest concern.

"I remember him dragging me to one side and saying to me, 'I've got big, big news. I might come into the office wearing an earring.' And I thought, 'OK, so what's the big news?' But for David, it was a huge thing. He wanted to take his time. He was always thinking of other people."

It may have been this selflessness that first brought Burgess into contact with the woman who now stands accused of his murder. Nina Kanagasingham is a 34-year-old Sri Lankan national currently awaiting gender reassignment surgery. No one is quite sure when their paths first crossed, although I am told by Sonia's friends that they may have met in a nightclub and Sonia, guided by her natural tendency to look after others, may have offered her help. Kanagasingham has now been remanded in custody pending her next appearance in court on 11 February.

"Sonia's defining characteristic was her kindness," says Tara. "It shone out of her. She had time for everybody and an absolute absence of snobbery and condescension. Sonia would interact equally comfortably and fluently with everyone without changing gear. I was devastated when Sonia died. I didn't sleep for days. The sheer suddenness and finality of it was what was so awful. I miss her terribly."

On 17 November, a funeral service for Sonia Burgess was held at St Martin-in-the-Fields, an impressive, grey-stone, porticoed church that overlooks Trafalgar Square. The church was filled with around 600 people from diverse backgrounds – lawyers, university contemporaries, former asylum seekers, members of the transgender community and countless others who, in some way, had had their lives touched by the person they knew as either David or Sonia. His three children stood up to deliver a eulogy about the father they had known, slipping easily between female and male pronouns as they talked. It was, everyone agreed, a moving tribute to an exceptional person.

As he sat in the pews casting his eye over the congregation, it struck Ian Baker as ironic that his friend would have been "utterly and acutely embarrassed by it all. He could never have imagined that so many people felt that way about him."

In the weeks and months after his friend's death, Baker has found it difficult to come to terms with David's absence. "You take people for granted in life. You don't stand back and think, 'How does he fit into the scheme of things?' Then, when he's gone, you suddenly realise." He looks down at the carpet for several seconds, his head tilted to one side, his fingers fidgeting. Eventually, he speaks. "You realise: I've never met a more compassionate person." And he sounds relieved, as though, after two hours' conversation, he has finally managed to find the right words.

This article was altered on 11/1/2011 to correct the medical term gender dysmorphia.