The Home Secretary is reported to have ordered them for his ex-lover's children. For the rich and famous they settle disputes where often huge sums are at stake. But paternity tests can wreak havoc on the life of a vulnerable child. Joanna Moorhead on a rapidly advancing science that has left the law in chaos

The girl is about six years old, and blonde: she is smiling, and her face is tilted upwards, her mouth open as a gloved hand advances with a medical swab. Glance at the picture quickly, and your first thought is that she's at the dentist's: that this is one of those images meant to reassure children that there's nothing to worry about, that she's in safe hands, that everything is fine.

The girl is about six years old, and blonde: she is smiling, and her face is tilted upwards, her mouth open as a gloved hand advances with a medical swab. Glance at the picture quickly, and your first thought is that she's at the dentist's: that this is one of those images meant to reassure children that there's nothing to worry about, that she's in safe hands, that everything is fine.

But everything is not fine for this little girl. The picture, on an internet site advertising paternity testing, does not tell the full story. Paternity testing is not the straightforward, risk-free procedure that this happy image suggests: instead, this little girl is at the centre of a complicated, probably acrimonious relationship. What's more, the check she's about to undergo could be about to make things a whole lot worse.

Paternity tests were thrust into the limelight last week after reports that the Home Secretary, David Blunkett, plans to insist on tests being carried out on two-year-old William Quinn, the son of his former lover Kimberly Fortier, and the baby she is expecting in February. Fortier, according to friends, opposes the test and wants to raise her children with her husband Stephen Quinn: whether the children are not his but Blunkett's, she'd rather not know.

Under UK law, paternity tests can be carried out without a mother's consent only if the father has legal guardianship of the child or children concerned, but Mr Blunkett could challenge that under an amendment to the Family Law Reform Act of 1969 passed in 2000. This gives the courts power to order paternity tests even where a mother refuses: and since the usual view of judges here is that it is always in a child's best interests to know his or her true parentage, even at the risk of disrupting the family unit, he could win his case.

But if the tests proved - as Blunkett presumably thinks they would - that he is the children's father, what would happen next? Would the Quinns continue to raise two children together despite only one being their natural parent? Would Mr Blunkett have contact rights? What would be the psychological and emotional effects upon not only a couple trying in incredibly difficult circumstances to piece their marriage back together, but also upon a man hurt by the break-up of a love affair, and, above all, upon two very small and vulnerable children?

It is an extraordinary irony that it is the Home Secretary's private life that is shedding new light on what is one of the fastest-changing barometers of life in modern Britain. Doubts over paternity are nothing new: as long as human beings have been having children, eyebrows have been raised continually over whether a particular child was fathered by a particular man, or not. But two things have changed: firstly, sexual relationships have become more complicated, with people tending to have more sexual partners, and more children are being born outside marriage. One in 10 of today's children is not related to the father they think is theirs, according to surveys. And over the past six years or so paternity testing - which used to be complicated and costly - has become simple, physically painless and, at around £150, relatively cheap. The result is a burgeoning paternity testing industry that's 10 times bigger than it was a decade ago. It's now worth £60m annually in the UK, is forecast to be worth £100m by 2008, and will continue to go up after that. "It's difficult to know the number of tests taking place in Britain each year," says David Hartshorne of the testing company Cellmark. "But we usually reckon on around 30,000 a year, and rising."

But has science overtaken society's obligation to provide for the emotional fallout caused by the easy availability and increased take-up of paternity testing? While judges take the view that it's always best for a child to know his or her true parentage, psychologists and counsellors don't concur. Their view is that, while it might one day be in a child's best interests to find out, it's not a given that it's always best to find out in infancy, or early childhood. Breaking up a family unit that could have provided a child with security could be a bigger psychological risk, they say, than bringing up a child in the belief that his or her father is someone who turns out, later in life, to be genetically unrelated.

Then there are issues around the practicalities of the tests themselves, and worries voiced by some over whether these are as stringent as they should be.

"You're only going to do one paternity test," says Denise Syndercombe Court, senior lecturer in haematology at St Bartholomew's and London Hospital School of Medicine. "There are always going to be mistakes in any medical test, but if it's a test for cancer and it's wrong at least there might be indications this was the case. With a paternity test it's a one-off and you live with the result - which is why it's so crucial that the test is as accurate as it could possibly be."

Does she believe the tests being carried out in the UK are not always accurate? She's not willing to be drawn on how widespread it could be, but says there's certainly room for corruption and errors, especially within testing companies that are not on the list drawn up recently by the Lord Chancellor of DNA-testing companies approved by his department.

"Some companies look impressive on their websites and there's lots of scientific explanation," she says. "But all I know is, I've had to re-do tests and sometimes we have got a different result from the one the original test came up with."

There have already been court cases that back up Syndercombe Court's fears: last year a man who substituted a friend's blood for his own to falsify a paternity test for the CSA was jailed for a year, and in September the managing director of a testing firm was sent to prison for three years after he faked the results of 150 tests. He was driven to his actions, the court heard, because he was swamped with work.

As the paternity testing industry grows, and the number of companies offering tests mushrooms, the problems of corruption and inaccuracy are only likely to get worse. Issues of consent, too, are a grey area and need to be looked at more closely: one idea outlined in a recent government White Paper would make it a criminal offence to carry out a test without proper parental consent.

Meanwhile the number of families affected by first uncertainty and then, perhaps, an uncomfortable result is likely to increase, and many thousands of people could find themselves in need of a listening ear to help them work through the worries that will undoubtedly be raised by these much more accurate tests.

Experts agree that this is an area in serious need of an injection of government initiative and action; and while it might be David Blunkett's tragedy that he's going to find out so much about the shortcomings of present practice, maybe this personal cloud will prove to have a public silver lining in the form of improved regulation in future.

Heartache or hope: the human side of what comes out of the laboratory

Avi Lasarow is managing director of DNA Bioscience, a London-based forensic science firm offering paternity tests

I set up DNA Bioscience two years ago, and over that time the number of paternity tests we do has tripled. We're now averaging 200 tests a month and I believe the figure will go on rising. People come to us for different reasons: you get paranoid fathers who wonder if the child they're bringing up really is theirs; you get mothers who aren't sure who the dad really is and want to know. Then there are the grannies who get in touch because they've never liked their daughter-in-law and wonder whether their son really is the grandchild's dad. Sometimes they ask us to do a test without parental consent. (We refuse.) And of course there are women needing to get proof for the CSA that this man really is their child's dad, and people needing proof of parentage for immigration purposes. Every case is unique: you could do this job for years and there would always be new stories to surprise you. Sometimes we do ask a potential client if they've considered the impact of the test result on their family; if not, they might want to think again. It doesn't make business sense to turn people away, but we're very aware that this is a field where people can easily get badly hurt.

Joe, 28, is a lorry driver who lives in Cheltenham

I met Sue around a year ago, at a work party. We got on well - we had a brief relationship, and then I went away on a long trip overseas. I was straight with her: I told her it wasn't anything serious. She said fine. She was on the Pill; it was just fun. But a few weeks ago, just after I'd got back from my travelling, I got home to find my mum in tears saying she'd had visitors: my ex-girlfriend and our six-week-old son! I was more shocked than I can describe - it literally took my breath away. I went round to see Sue and there was this gorgeous little boy - he's marvellous, but I couldn't believe he could be mine. It had all been so fleeting, so baggage free. I had to find out for sure, otherwise there would always have been that doubt, and if push came to shove maybe I'd have disappeared out of his life because I wasn't certain he was mine. So we organised a test. It was a shock again, getting the news that the baby was certainly mine, but I'm glad we did it. Now I know for sure. I've been in tears a lot the past few days, and I think it's unfair I've ended up in this situation, but I know the baby is my son and that does means something.

Phillip Hodson is a psychotherapist and a fellow of the British Association of Counselling and Psychotherapy

For every paternity test that takes place, there's going to be a complicated situation. If you're a woman having an abortion or a couple seeking fertility treatment, you're offered counselling. But if you're involved in a paternity test, that's not routinely suggested, and I think it should be. Sometimes, a person might want to think again about whether to have the test at all. The overriding need in every case has to be the welfare of the child, and sometimes a paternity test isn't about that at all. In the Blunkett case, for example, there are real reasons to wonder whether this isn't about the paternity issue as much as it's about being rejected by a former lover. In other cases there are men who might need help to see their responsibilities, and counselling can also help women to see why their ex-partners are behaving in a particular way, and how it might not be just about hostility.

Beth, 32, is a full-time mother who lives near Hull

Peter and I went out for four years before things started to go wrong and we split up. I met Nick a few months later. We had a whirlwind romance and got married. But within a few months he was getting drunk, going out all the time, hitting me. I was at my lowest point when I saw Peter again; straight away we clicked. He felt the same: we both knew my marriage was a mistake. I left Nick within a few weeks of meeting Peter again, but at around the same time I found I was pregnant. Peter was so pleased, because he assumed the baby must be his. I thought it probably was his, but there was this doubt in my mind that it could be Nick's. I put it to the back of my mind until, seven months after our son was born, Nick phoned up to say he believed he was the dad and that he was going to see a solicitor about his access rights. Peter and I were devastated. After much soul-searching we decided on a test, which Nick also agreed to. I honestly don't know how I got through the days when we were waiting for the result. When the letter came we were literally screaming and crying with joy to find our baby was Peter's.

Anna Tang is a genetic profiler at Complement Genomics, a paternity testing laboratory in Sunderland

Doing a paternity test is relatively straightforward. Cells are taken from the inside of the mouth on a swab, put into sealed packets and sent to us. We rehydrate the cells and use an enzyme to separate the DNA from the rest of the cell. We then amplify 16 regions of the DNA so we can compare the different samples we've got. It improves the accuracy of the result if we've got the mother's DNA as well as the child's and the potential father's. Accuracy is very important and we do lots of checks at each stage, but the basic test is comparing the samples with one another once the DNA is amplified to see whether or not they match up. It's a scientific process but you're always aware of the human side. It's most difficult when the child isn't a baby but an older child who's aware of what's happening and is going to be deeply affected by the result.