Louise McLoughlin was just 13 when the very foundations of her happy childhood were ripped from under her. Her parents sat her down one day and told her they had something important to tell her.

They revealed that her dad — the man who'd loved her, hugged away her tantrums, wiped away her tears and kissed her goodnight — might not be her father after all.

She'd been conceived through IVF, her parents explained. They had struggled to conceive for years and, to increase their chances, they'd mixed her dad's sperm with that of a donor, meaning there was a 50 per cent chance he was not biologically related to her.

Picture shows a close up of female scientist looking to microscope in clinical laboratory over hydrogen chemical formula and DNA molecule structure

They had always planned to tell her, and figured that at 13 she was old enough to know the truth.

'That night, I slept in my parents' bed, between them, sobbing,' recalls Louise. 'At an age when I was just figuring out who I was, half of my identity had been torn away.'

Haunted by uncertainty, two years later, aged 15, Louise insisted on a DNA test. 'The results confirming my worst fears arrived when Dad was at work,' she recalls. 'I opened the letter and then waited by the front door, announcing the moment he walked through it: 'We got the DNA results. It's not the news we wanted.'

'We hugged in the hall and he held me, while I sobbed, for a long time afterwards.'

Nicki Field (second right) discovered when she was 13 that the man she believed was her father was not. She is pictured with her mother and stepfather

In the years that followed, while her love for her dad remained steadfast, Louise, now a 27-year-old TV producer from London, felt there had been a seismic shift in how she viewed her place in the family, and indeed herself.

'I'd spend long periods in front of the mirror, figuring out which of my features — brown hair, blue eyes, freckles, button nose — I'd inherited from my biological father. I used to fantasise about him being a famous guitarist and songwriter.'

While this may sound more like the plotline from a soap opera than everyday life, Louise's family is far from the only one left reeling by advances in medical science, which mean that secrets which would previously have gone undiscovered — or, at the very least, unproven — are now easily exposed.

In this brave new world, by paying £50 to a DNA-testing clinic and sending off a saliva swab in the post, it is possible to ascertain who each of us is, and is not, related to.

Ancestry websites have sprung up, too, where people can post their DNA profiles to be matched to others all over the world, revealing long-lost relatives and dropping almighty bombshells without warning.

While it is impossible to be exact, experts estimate that between 2 and 4 per cent of the population — that's up to 2.6 million people in the UK — believes one of their parents (usually the father, for obvious biological reasons) is not actually related to them.

The implications are huge, both on a personal and societal level. Recently, millionaire Richard Mason, co-founder of comparison website Moneysupermarket.com, revealed how he'd been left 'in pieces' after discovering his three grown-up sons were not biologically his.

He'd been diagnosed with the life-limiting medical condition cystic fibrosis, and was told it was not physically possible for him to father children. He then sued his former wife, Kate, who has been ordered to repay £250,000 of her £4 million divorce settlement.

Richard's sons — a 23-year-old and 19-year-old twins, two of whom refuse to have any contact with him, were no doubt as distraught as their father at the news.

Nicki Field (pictured) was devastated and it was many years before she traced the family of the man her mother told her was, in fact, her biological father

One of the twins, Joel Mason, says he doesn't know who his biological father is, and doesn't care. 'Dad is a very manipulative man, not the kind of person you want to be with, but he's still my dad and I'm not going to look for my real one,' he explained.

Kirsty Ward, a 28-year-old mother of two from London, saw her childhood similarly ripped apart 14 years ago by an envelope marked 'private and confidential'. A DNA test revealed her dad had fathered a baby with her 19-year-old cousin from her mother's side of the family.

Within months, Kirsty's parents had divorced and a huge rift opened in the family, which has never healed.

'Even now I feel sick when I think about the day the results came through,' says Kirsty. 'I was sent to my older sister's house while my parents opened the letter, and the moment Mum called I knew from my sister's face that our lives would never be the same. Dad was packing his bags by the time I got home.'

The baby, it turned out, had been conceived after Kirsty's father, then 38, and her cousin had unexpectedly met up during a night out.

In the throes of labour, her cousin had tearfully revealed the identity of her baby's father, although at first Kirsty's mum — following reassurances from her husband — insisted this could not be true. A few months later, however, in an attempt to pour cold water on her niece's continuing claims about her baby's parentage, Kirsty's mum, a teacher, insisted her husband take a DNA test.

The impact of the result on teenage Kirsty was profound. 'I was desperately unhappy for many years and didn't feel I could trust anyone, which put a huge strain on friendships and relationships,' she says.

'Nor could I speak to anyone in my family about how upset I was because we were all hurting,' says Kirsty. 'My cousin and her family lived five doors away from us, though they moved away after all this, so we had grown up together and were very close.'

Recently, Kirsty's half-sister, who is now 14, got in touch with her and the two have managed to build a relationship, despite the past rift.

Recently, millionaire Richard Mason, co-founder of comparison website Moneysupermarket.com, revealed how he'd been left 'in pieces' after discovering his three grown-up sons (who he is pictured with) were not biologically his.

Theirs is a pain with which Nicki Field, a teacher who, at 50, has just discovered the identity of her biological father, can certainly empathise.

Hers has been a long and agonising journey since the day, 37 years ago — her 13th birthday — when Nicki's mother revealed that the man she had thought was her dad was not.

'He'd bought me a watch and, overcome with joy, I said to Mum: 'It's the best birthday present ever — and he is the best dad in the world!' She looked at me, then said: 'He's not even your real dad,' ' recalls Nicki.

'The shock was so overwhelming, a punch in the face would have been less painful.

'They split up six months later and I realise now that the marriage was already on the rocks, so she saw no point in carrying on the pretence.'

Nicki's mother had met her stepfather when Nicki was two years old — the couple went on to have two daughters of their own.

Nicki's biological father, her mum told her, was called Gary Holmes and lived in London. In the days long before there were Google searches and genealogy databases, Nicki had no idea how to trace him.

Fast forward three decades to 2014, however, and the married mother of four children, aged between ten and 28, went searching.

She managed to find contact details for a brother of Gary Holmes, who, it emerged, had died in 1993.

'Kirsty Ward, a 28-year-old mother of two from London, saw her childhood similarly ripped apart 14 years ago by an envelope marked 'private and confidential'. A DNA test revealed her dad had fathered a baby with her 19-year-old cousin from her mother's side of the family'. She is pictured aged 14

Gary had carried a photograph of Nicki until his dying day but, while his family were very welcoming, DNA tests showed she was not, in fact, related to them.

Her mother insisted the test must be wrong, but Nicki, trusting the science, knew otherwise. 'I was so angry with my mum for repeatedly lying to me that I haven't had any contact with her since,' says Nicki, from Kimbolton, Cambridgeshire.

'She never made any attempt to explain or apologise, so I've no idea why she told me my father was one man when he turns out to be someone different. She must have been sleeping with them both.'

Nicki sent her DNA for testing to genealogy firm Ancestry.co.uk, and posted the results on sites which match people with shared genes.

Finally, in July 2017, she was alerted to someone who was said to be her first cousin. Further investigations revealed that his uncle, a married dad of four, was Nicki's biological father.

There was more heartache, however, when the man refused to acknowledge that he'd had an affair with Nicki's mother, or that she is his daughter.

'It was devastating to be told neither he, nor any of his children, with whom I share an obvious resemblance, wanted me in their lives,' says Nicki.

'My mother is black and racism was rife in the Sixties, so, if he knew I existed, that may be the reason he didn't want to know me.

'Most people don't realise the power of DNA tests and the can of worms they can open,' she adds.

'But there are thousands of us in online support groups, such as NPE (Not Parent Expected), who are all too aware. Every day heartbroken members show up saying: 'I've just found out through a DNA test that my dad is not my father.' This deceit destroys lives.'

Consultant clinical psychologist Emma Citron, who has counselled patients through the aftermath of this thoroughly modern trauma, agrees it can lead to trust and rejection issues, which can make future relationships untenable.

She also fears the numbers affected could soon escalate.

'One of the most popular gifts in the U.S. at Christmas was a £20 DNA test, which means a lot of people will now be processing devastating revelations about their parentage,' she says. 'As with so many trends over there, we're highly likely to see it take off here.

'Those who deceive children about their parentage always say they do so 'with the best intentions', but it can lead to apocalyptic moments which throw their whole lives into turmoil and leave them questioning everything about themselves.'

Mandy Booth, 54, had her own 'apocalyptic moment' three years ago when a woman approached her in Sainsbury's and announced that she was her sister.

Still living in the small rural town in Yorkshire where she had grown up, Mandy knew the woman vaguely — she had been in the same class at school as her younger brother — but was baffled by her claim that Mandy's mother had had a fling with this woman's father, and Mandy was the result.

Shaken and confused, she headed home and called her mum, who flatly denied the story, insisting that her biological father was the man she had separated from when Mandy and her elder sister were small and whom she had seen only a handful of times since.

However, when her mother died several months later, Mandy signed up to the DNA testing site 23andme.com and was contacted by a sibling of the woman she had met in the supermarket.

Confirmation that Mandy is indeed their half-sister, and their father's daughter, came through just a few days after he, too, died, by then in his 80s.

'I would have liked the chance to ask him why he'd never told me I was his daughter, because he clearly knew — my aunt told his wife that my mum was pregnant with me,' she says.

'I also look just like him, the same bone structure and hazel eyes, as do my daughter and my granddaughter.

'I feel sorry for my mother that she had to lie, but angry, too. My biological father had three sons, including the one in my class, and I shudder to think that I could have unwittingly ended up in a relationship with one of them.

'I know I, and others in this position, have been described as 'a dirty little secret' and that leads to intense feelings of shame, which we don't deserve.'

After the test revealed that her cousin's new baby had, in fact, been fathered by her father Kirsty Ward's parents immediately separated and divorced soon afterwards

In an era when a drop of saliva, flake of skin or strand of hair can instantly reveal a person's identity, as well as their kith and kin — without any of the attendant genetics counselling the NHS provides when testing for hereditary diseases — it seems impossible to legislate for the fall-out.

Louise McLoughlin's story has a happy ending, though. In 2012, supported by a group which matches donor-conceived adults to their biological family, Louise decided to take a DNA test.

Though there has been no match with her father, in 2017, a woman eight months Louise's senior got in touch and, to her delight, turned out to be her half-sister. Her mother had been inseminated with sperm from the same donor.

'We meet up often and message every day,' Louise says. 'I hope that she'll be in my life for ever.'