One evening in 1988, a man broke into a house in Canterbury. He awoke an 11-year-old girl in her bed and raped her, threatening her with a knife if she screamed. He then indecently assaulted her nine-year-old sister. Earlier, their mother had left home to work a night shift for the first time. After the attacks, the girls ran in tears to their mother's workplace.

For 13 years the case remained unsolved. Then in March 2001, John Wood, a 59-year-old man with previous convictions for sex offences, was arrested for shoplifting in Derby. Detectives took his mouth swab and checked it against the national DNA database of more than 5,000 unsolved crimes. They found it matched semen on sheets preserved after the attacks on the two sisters. In June that year Wood pleaded guilty and was sentenced to 15 years' imprisonment. After his conviction, the mother said: "I don't know how we've coped as a family and how we got through it, but we felt he would never be caught."

This was no doubt just the kind of case Tony Blair had in mind when he visited the Forensic Science Service's London headquarters this week and eulogised the DNA Database as a crime-busting tool. He called for the national DNA database to be expanded to include every citizen. While at the centre, he heard how advances in DNA technology are not only boosting detection rates in current cases but also helping police in reviews of so-called cold cases - some several decades old - where there was no new evidence.
Paul Hackett, the DNA manager for the Forensic Science Service, says: "I was trying all the while to give Tony Blair the wow factor about what we do." When Hackett explains some of the FSS's latest DNA profiling techniques, it is hard not to be wowed. For a long time, DNA "fingerprinting" needed at least 200 cells to be effective. Now, thanks to a technique called "low copy number" (LCN) profiling, a single cell might be enough.

"In the past we would have to have a visible trace - somebody would have had to have cut themselves and left blood at the scene, for instance," says Hackett. "Now ... we can get a profile from, say, an ear print against a glass window. What's more, when there are mixed samples of DNA we can increasingly separate them with LCN profiling. So that means a homeowner's sample on a vacuum cleaner can be distinguished from that of the burglar, for example.

"Ten years ago no one would have thought we could get DNA profiles from a single cell, still less from a sample that we couldn't see. We've come a long way."

For instance, in March the so-called Ripper Hoaxer (aka Wearside Jack) was jailed for eight years for perverting the course of justice by sending three hoax letters and making phone calls claiming to be the serial killer known as the Yorkshire Ripper in the late 1970s. His hoaxes put detectives on the wrong trail and delayed the conviction of Peter Sutcliffe by several years, during which time he murdered three more women. Sutcliffe was jailed in 1981, but the identity of Wearside Jack remained a mystery for more than a quarter of a century until the FSS analysed a portion of one of the envelopes using the then new technique of LCN profiling. Tests yielded a DNA profile that produced a single hit - a man in Northumbria called John Humble, who ultimately pleaded guilty to the charge.

Such is British pre-eminence in DNA profiling techniques that FSS skills are in demand abroad. An FSS analysis of the knife used to kill the Swedish foreign minister Anna Lindh in 2003 was instrumental in securing the conviction of her murderer. Last year, the FSS was requested by Australian authorities to do DNA tests on hand ties used to restrain Joanne Lees and on swabs taken from a gear stick - these tests helped secure the conviction of Bradley Murdoch for the murder of Peter Falconio.

Hackett believes it will not be long before police officers will be equipped with handheld devices that can transmit samples to the DNA Database computer in Birmingham. "They will be analogous to roadside breathalysers," he says. "A great deal of our work now isn't so much in the lab ... but in developing software and other techniques that can enable us to be quicker and more responsive at scenes of crime." He also hopes that, from a single cell left at the scene of a crime, the FSS will one day be able to build a 3D photofit of a suspect that includes their hair and eye colours, height, skin type, and maybe even a personality profile.

Perhaps wowed by such scientific advances and Hackett's crystal-ball gazing, the prime minister on Monday dismissed the ethical misgivings of MPs and human rights groups. Many of these objectors worry about the many innocent British citizens, as well as 51,000 juveniles who have been arrested and then freed without charge, who already have DNA profiles on the database. They fear that expanding it will raise even graver civil liberties issues and risk miscarriages of justice. The Tory MP Grant Schapps investigated the case of a teenager whose DNA data was retained on the database even though his arrest was a case of mistaken identity. "The police refused to remove the DNA profile even though they realised they had picked up the wrong chap," he said.

It is this kind of case that makes Shami Chakrabarti, director of Liberty, sceptical about Blair's dream of extending the DNA database. "The project of turning a nation of citizens into one of suspects may well be the legacy sought by the prime minister," she says. "However, a smaller, more manageable DNA database of those convicted of serious sexual and violent offences would be a speedier crime-fighting tool and cost less to our purses and privacy." The Tories have called for a parliamentary vote on whether details of people who were innocent or not charged should be included against their wishes. But during his visit Blair said the public backed the expansion of the database because it was "helping us to track down murderers [and] rapists".

The prime minister was echoing his former home secretary David Blunkett, who recently pointed out that last year 20,000 people were convicted with the help of DNA. The crimes included 422 murders and manslaughters, 645 rapes and 9,000 domestic burglaries, he wrote. In a column for the Sun last month, Blunkett claimed that the introduction of the DNA Database in 1995 "met complete opposition from large swaths of politicians and still faces continuing hostility from the human rights lobby. They say it is an 'intrusion'. Well, it certainly is. An intrusion into the arrogant confidence of criminals who think they can cock a snook at society and walk free to carry out other crimes which would otherwise go undetected."

No doubt what neither Blair nor Blunkett have in mind when they champion the database is the case of Raymond Easton. The Swindon man was arrested in 1999 for a burglary in Bolton on the basis that a DNA sample found at the scene of the crime matched his own on six points - what police called a "37 million to one chance". Easton was not a criminal seeking to cock a snook at anybody. He told police that he had never been to Bolton, that he had Parkinson's disease so advanced that he could barely dress himself, still less commit a burglary. His DNA had been taken after a family dispute and had been held on the database, but after the samples were compared more rigorously at a further four points of identification and failed to match, the case was thrown out of court.

Easton is the leading British case of a DNA mismatch, but according to Professor Allan Jamieson, director of Edinburgh's Forensic Institute, more such cases are possible. "I fear innocent people will be mistakenly identified as suspects as a consequence of being on this database," he says. Jamieson's worry is that some of the new supersensitive DNA techniques, such as LCN, may make such mistakes more likely. He is also concerned that DNA evidence is too often regarded as definitive in determining guilt in court.

Hackett is sympathetic to Jamieson's fears. "There is a risk of mis
takes, but the risk isn't really with mismatching. It's about how the background information from crime scenes is interpreted. The other question is how clean you keep the labs, and we do so scrupulously to minimise the risk of contamination." He is contemptuous of the suggestion that the new techniques provide greater opportunities for police abuses.

The British national DNA database is the biggest in the world. According to the Home Office, the UK has 3,130,429 DNA samples on the national database, 5.23% of the population. The second largest is Austria's database, which has 84,379 samples (1.04% of its population). The US has 2,941,206 DNA samples (0.99%). The majority of EU countries retain fewer than 100,000 samples on their DNA databases.

Britain's was also the world's first such database. "It was established in 1995 because of two factors," says Hackett. "First, there was a sea change in the technology. We had just moved to something called SGM profiling that allowed us to take very small samples, such as mouth swabs. At the same time, there was the political will to set up a database because its potential for helping solve crimes was becoming more apparent."

Since 1995, Hackett and other FSS scientists have been responsible for developing other DNA profiling techniques. One recently developed technique, "familial searching", can help identify an offender when their DNA profile is not on the database. It has been used successfully in a number of cases, the first of which resulted in the manslaughter conviction of a man who had thrown a brick through a lorry driver's cab windscreen.

Another new technique, DNA Boost, was launched as a pilot this month in four police forces in northern England. It involves using computer-based analysis to interpret DNA samples when a surface has been touched by more than one person, or where only small amounts or poor quality material has been left. "This means a great many more families can look forward to securing justice," says Hackett.

But as techniques have advanced, so has disquiet about the rise in the number of people who are profiled on the DNA Database and what that information is used for. Over the past five years, legislation has permitted police to take and keep many more DNA samples. The Criminal Justice Act 2001 permitted police to take DNA samples from everyone charged (previously the person had to be convicted). The Criminal Justice Act 2003 allowed police to take and keep the DNA of anyone arrested for an imprisonable offence, even if they are not charged. These changes led Helen Wallace, the deputy director of GeneWatch UK, to say: "Britain's DNA Database is spiralling out of control." Genewatch has launched a campaign called Reclaim Your DNA! to have innocent adults and juveniles' DNA profiles removed from the database. At present, Wallace points out, that decision is at the discretion of the relevant chief constable. Liberty is representing several people taking legal action to have their profiles removed.

But why bother? As Lady Helena Kennedy, chairwoman of the human genetics commission, asks: "Why should we be alarmed that police or other investigators might have sight of our private records if we are decent law-abiding folk?" One answer, Kennedy says, is that "being on a database of potential offenders which might be regularly trawled by the police means that one is on a list of suspects and that surely very subtly alters the way in which the state sees, and we see, our fellow citizens". Another is that there is a risk of such information being used by other parties. Genewatch claims the commercial company LGC, which analyses some DNA samples for the police, has retained its own "mini-database". A third is that ethnic minorities will be disproportionately affected if the DNA database is enlarged. "Huge numbers of people picked up in their youth but acquitted of any crime will remain on the database for life," writes Kennedy in her book Just Law.

The number of innocent Britons whose DNA profile remains on the database appalls Kennedy. "This takes Britain to the top of the illiberal league: nowhere else in the free world is this happening," she writes. Kennedy points out that Canada and France have already legislated to prevent the retention of samples from people acquitted of crimes and also that samples from juvenile offenders will be destroyed once they reach adulthood so long as they commit no more crimes during a set period. The FBI, she claims, has "expressed jealous amazement" at Britain's "hoarding of DNA", underlying which "seems to be the cynical belief that those who are suspected of a crime are probably guilty, even if acquitted, and likely to be involved in further offending".

Britain, then, may not just be leading the world in the excellence of its DNA profiling techniques, but also in using those techniques to curtail rights. Kennedy writes: "[The] American public would find such inroads into civil liberties wholly unacceptable despite the heat of their feelings about crime control." In Britain, it seems, we have less compunction.