Helen Steel, with fellow activist David Morris, in London in 1990 during the McLibel trial. During their brief conversation at the airport, Dines apologised "unreservedly" for the way he had treated her. But it was not enough to mollify his former partner. "He talked to me about how, when he had been sent in to infiltrate groups in north London, it was to look out for what they call extremists," said Steel. "It's a vague term, never properly defined, that seems to include anyone who challenges people in positions of power." Lured into relationships with undercover police officers I first met Helen Steel 20 years ago, when she was a defendant in what became known as the McLibel case. She and fellow members of London Greenpeace had distributed leaflets accusing McDonald's of, among other things, contributing to heart disease, cruelty to animals and destruction of rainforest. When libel writs were served on the campaigners, Steel and fellow activist David Morris elected to tough it out in court in what would become the longest running civil case in the history of British justice and a PR disaster for the burger chain. Steel was 26 then, an elfin figure with a curtain of dark hair and the stubbornness of a truthful child. In our interview I asked what drove her to take on a global corporation and its $8000-a-day legal team.

"Anger," she said, "I was angry." Love hurts: Mark Kennedy, known as Mark Stone to “Lisa” during their six-year relationship. I recently asked Steel how she felt when she discovered John Dines' true identity. "I still had this idea that maybe his love had been genuine," she says. What if the man you loved, who had shared your life and your bed for years, turned out not to exist? What if everything you thought you knew about him was a lie and the charismatic man you fell for, who shared your political ideals, was someone else entirely? A man with a different name and a wife and children waiting at home for him, a spy paid for every hour he spent with you and your friends and your family, whose employers monitored your most intimate texts and conversations and who, one day, vanished without trace? Helen Steel wasn't the only woman who found herself in this situation. Essentially, this is what happened to at least seven other British women who were lured into relationships with undercover police officers over the course of 25 years. The women had their hearts broken when their lovers mysteriously disappeared, apparently in a state of emotional turmoil. Two of the officers had fathered children with their targets; some of the women spent years trying to track down men who they believed were in great distress.

Only later did the group of women learn that demonstrating a nervous breakdown is a standard exit strategy for officers at the end of a deployment. "We all know that now," one woman told me. "But at the time you're just frantic with worry about the person you love – and you think it's only happened to you." 'He did this thing called mirroring' I am in the upstairs cafe of a converted waterfront warehouse looking for a woman in her early 40s with red hair. Lisa Jones is not her real name - she asks me to simply call her Lisa and not to identify the city. She moved here to start a new life after her partner of almost seven years, the man she knew as Mark Stone, was exposed as Mark Kennedy, a police spy. She says the experience turned her life upside down and she is only now ready to tell her story. Lisa has been involved in the environmental movement for most of her adult life. In 2003, when she met Stone, she was active in the anti-capitalist movement and in campaigns on climate change. He expressed interest in the causes she cared about and was soon part of her group of activist friends, attending meetings and giving people lifts in his car. Was she attracted to him? She nods. "A bit. He had a disarming manner and he was very good at making friends. When you talked to him, he made you feel you were the only person in the world at that moment.

"He did this thing called mirroring: if you said you were into a particular band, for example, he would say he liked them, too, and would know something about them. He got involved with my social group and started coming on climbing holidays with us." They began a relationship. "Six months in, it became serious," says Lisa. "We were very openly a couple. We went on demonstrations, shared a tent and were arrested together in Germany. "He met my family and supported me through my father's illness." When her father died, Stone travelled in the mourners' car with Lisa. "That night he held me while I cried. He was the closest person in the world to me and I thought I knew him better than anyone." Stone never introduced her to his own friends or family, "but he always had a good story for why. The one person he did talk about was a brother living in America who I spoke to a few times when he phoned." Lisa later learnt the man was Kennedy's real brother. (Officers were advised to pick a few things about their lives that were true and could be authenticated without putting them at risk.) From the beginning Stone had told everyone that he worked all over the country as an industrial climber, which explained why he always had money. His cover story also accounted for his frequent absences. "I overlooked his erratic schedule … not there when I needed him, turning up when I didn't expect him, jobs booked or cancelled at the last minute," says Lisa.

Six years into their relationship, in October 2009, Stone told Lisa that he needed to get away. He said his house had been raided by police after a demonstration and he was spooked. "He was not himself," she says. "It really felt to me that I was seeing him through some sort of breakdown. He leant on me very heavily." 'It sounds ludicrous but I wanted to believe him' Stone said he was going to stay with his brother in America. He had sold his car, apparently quit his job and cleared out his house. It looked very much as if he wouldn't be coming back. But three months later he reappeared. "He was different, quite jumpy, but I was so happy to see him," says Lisa. In July 2010, the couple took their van to the Italian Alps for a climbing holiday. One morning while Stone was out cycling, Lisa found a passport in the glove box. It was in the name of Mark Kennedy and included a child's details. Rooting around some more, she found a mobile phone containing emails from two children addressed to "Dad". What happened when Stone returned? "I didn't say anything," she says. "I felt sick and didn't sleep for three days, trying to work up the courage to ask him about it. I was scared of what I might find out."

When she did challenge him, he broke down. "He cried and cried. He'd always said that he had a dodgy past in the drugs trade and that was why he'd wanted to start a new life with people like us who cared about the world. He told me that one day a drug deal had gone wrong, his partner had been shot dead and he'd taken that guy's kids under his wing, bringing them up as his own children and taking their mum's name." She looks at me, lifts a hand and lets it drop. "I know it sounds ludicrous but the way he told it, I wanted to believe him." That same summer they went climbing again. "We did an amazing trip in the Dolomites, in Italy. It felt totally blissful and we were on such a high. I'd convinced myself everything was fine but when we got back home and he went to work I got a bit of perspective." She asked a friend who was into genealogy to do a search for the name on the passport. "In a short space of time we had a marriage certificate and children's birth certificates, both listing his occupation as a police officer." Mark Kennedy, they soon discovered, had a home and a family in Ireland. Lisa rang him and asked him to come home to answer some questions. He arrived in the early hours of the following morning and sat on sofas with Lisa and four of their close friends. "We genuinely wanted to give him a chance to explain himself," she says. At first he prevaricated but they pressed him gently, saying, "Tell us who you are." He eventually admitted he had joined the police 20 years earlier and had worked undercover in the drugs trade before being recruited to a secret unit which sent officers to infiltrate groups of political dissidents. He was not the only one living a double life. "There are lots of us," he told them.

Fake relationships gave acceptance by association Lisa's brother had become friendly with Kennedy during the years the couple were together and a few days after the policeman's admissions to Lisa and her friends, he called her. "What's wrong with Mark?" he asked. "I can't hear what he's saying through all the sobbing. He's threatening to kill himself." Months later, Lisa agreed to meet her ex-partner. "I wanted to tell him what I thought of him," she says. "I had questions – what did it mean when this happened or that happened? But he was a mess and I realised I would never be able to trust his answers." It turned out that when her partner had exhibited signs of a breakdown and disappeared to America, it was because his deployment was ending and he needed to extricate himself. "But then, no longer a policeman, he came back to his life with me," says Lisa. "I don't know why. Was it because he wanted his old life back or just that he didn't know what else to do?"

Or because he loved her? She shrugs. "That's what I'd like to think, but I'll never really know." Does she think he deliberately left that passport for her to find? "Yeah, I do, and I often wonder if he'd told me the truth then, and said he would leave the police and we'd start a new life, I honestly don't know what I'd have done. But now I think of the times I held him in my arms while he cried and told me lies. I think I was saved by those lies." Subsequently, in January 2011, Mark Kennedy sold his story to a tabloid newspaper in which he said he had loved the red-haired Welsh woman he'd spent six years of his life with and regretted hurting her. "He said he would never have informed on me," she says. "The main thing he got from me – the point of all these fake relationships – is acceptance by association." She gave him legitimacy among her fellow activists? "Exactly," says Lisa.

Undercover for years at a time Soon after Kennedy was unmasked, Lisa received a letter from a woman who said something similar had happened to her. It was Helen Steel. "When you're ready I'd like to talk," she wrote. Lisa had not made a complaint or revealed her identity but connections were being made through the activist community. One by one, other women came forward to share their suspicions about men they had had relationships with, men who had inexplicably disappeared. They learnt more about the intrusive surveillance habits of the special demonstration squad. Officers posing as social rebels, nicknamed "the hairies" by colleagues, went undercover for years at a time, joining campaigners at meetings and demonstrations and feeding information back to their unit. At least six were known to have had long-term, intimate relationships with female activists. Dates and places, real names and false identities were established. Within a year, in December 2011, eight of the women initiated legal action against the Met for violation of their human rights and offences under common law.

They came up against a wall of silence. The police relied on secrecy, the protection of covert human intelligence sources (CHIS) and the well-worn practice of all secret services to "neither confirm nor deny" any alleged undercover activity. The Met was challenged by the women's lawyers in court hearings and appeals. But the women themselves never got a chance to make their claims in court and the police never had to defend those claims. "It was deeply frustrating and insulting for the women," says their lawyer Harriet Wistrich. "We pushed as hard as we could to get some form of disclosure. What all these women want is to know what was reported back about them, what exactly was going on in these significant periods of their lives. All the information they have is the result of their own detective work. The police have disclosed nothing." 'Abusive, deceitful, manipulative' relationships To establish their claims against the police, Steel and her fellow litigants were required to undergo psychological assessment. This, she says, felt like adding insult to injury. "You couldn't just say, 'This was upsetting and wasted this many years of my life', " Steel explains. "You had to demonstrate either financial loss or a diagnosable psychological injury. If you've lost the opportunity to have children or the opportunity to form loving relationships, that's not counted as loss because you can't put a value on it.

"So you have to relive everything, you have to see it all written down, all intensely personal. You have to focus on everything that's gone wrong and you end up feeling you've made a failure of your life." It would take four years of legal wrangling, court hearings, appeals and mediation to wring out a result: a settlement, in the form of financial compensation, with the amounts confidential but said to be "substantial," and an apology from the Met's Commissioner, Bernard Hogan-Howe. The behaviour of certain undercover police officers was totally unacceptable, he finally admitted in January this year. "[They] entered into long-term, intimate sexual relationships with women which were abusive, deceitful, manipulative and wrong." Neither Lisa nor Helen Steel strikes me as naive or gullible. On the contrary, they are gutsy women who have stood up for the things they believed in. They were targeted by men adept at role play and tutored in the art of seduction and betrayal, personable men who it felt good to have on side. And they were men alert, like all good spies, for the signal to get out. They left havoc in their wake. Married officers picked for covert work

Not all undercover officers survived deployment in good shape. Only married officers were picked for covert work because it would give them stability but several of those marriages have since broken up. Researching their book Undercover: The True Story of Britain's Secret Police, journalists Rob Evans and Paul Lewis heard from insiders about the strains of living a double life, of experiencing brutality from fellow officers at demonstrations, of feeling guilt about the activist friends they had come to respect and like and even loved: it all took a toll. Two officers suffered post-traumatic stress disorder, sued their former employers and were eventually paid compensation. One of them told Evans and Lewis that of the 10 covert officers he had served alongside, six had experienced psychological problems. Mark Kennedy left the police and told the tabloid newspaper that he had had no support from his employers. His only friends were activists and he had suicidal thoughts. The police apology to the women for the actions of these officers runs to a fulsome and contrite two pages but nowhere does it admit that the officers had been authorised to form sexual relationships with their targets. It simply says that oversight was lacking. "This is a question we want an answer to," says Helen Steel. "How far was this behaviour sanctioned by police bosses and therefore, ultimately, by the state?" An inquiry will begin hearing evidence in July. Led by Lord Justice Pitchford, it promises to "explore the state of awareness of undercover policing within Her Majesty's Government". It will also look at questions of "oversight, justification and authorisation" and will investigate what one MP has called the "ghoulish practice" of giving undercover officers the identities of dead children. The scope of the inquiry is broad – there are almost 200 "core participants" whose lives have been affected by undercover policing – but how much of the evidence will actually be public? The Metropolitan Police has argued that significant portions of the inquiry should sit in secret. "Undercover police officers and their families are likely to face real harm if anything is disclosed that tends to identify them," claims the Met. "[Officers] will suffer the unfairness of losing a lifelong expectation that their roles would not be made public."

Lawyers, both for the women and for other groups affected by undercover policing, are building their cases against the secrecy. Justice Pitchford is expected to respond to both sides on the issue later this month. Helen Steel is outraged. "Why should those who committed the abuses be protected above those who suffered the abuses? If all the evidence for the police, state and Home Office is heard in private, well, basically it will be a complete whitewash." 'The field of people you can trust is very small' Steel has lost none of the fighting spirit she showed when I first met her all those years ago. Why is she such a battler? She blinks, then laughs. "Blimey, that's some question. If I see injustice, I don't like to just accept it." Presumably the long search for her former partner had left little room for other relationships? "No, nothing from 1992 when he left until 2004," she says. "The field of people you can trust is very small." She frowns. "I'm not sure how much I want to talk about this." But she did then meet someone? "By this stage I really wanted to have children. I got into a relationship with a close friend who I'd known a long time and trusted." How long were they together? "Six years, but I had a series of miscarriages." That's very hard on a relationship, I say. She nods and stares ahead; her eyes fill with tears.

Lisa, too, lost the chance to have children. She and Kennedy were together for most of her 30s and so, I suggest to her, he effectively stole her childbearing years. "Yes, and I feel very angry," she says. "Who knows who I might have met, what decisions I might have made, but those choices were taken from me by the state." She hasn't met anyone else and says it is hard to think about a new relationship. "The whole world is doing internet dating and there's no way I'd contemplate that. The next man in my life is going to need a cast-iron set of references."