Without the miracle of DNA profiling, two depraved killers would still be prowling the streets of Ipswich and London this weekend, ready to murder for no other reason than sexual depravity or hatred of women. Mark Dixie, a pub chef, was first linked to the savage murder of Sally Anne Bowman, an aspiring model, purely as a result of DNA evidence. It involved a large slice of luck; had it not been for his involvement in a pub brawl a few months after the murder, his DNA would not have been on the national database.

Steve Wright, the serial killer of five prostitutes in Ipswich, was also caught as a result of DNA matches. Had he not been arrested and convicted for a minor offence five years ago - stealing a modest sum of money from the hotel where he worked as a barman - his DNA would not have been on record. He had not been a suspect for Suffolk police, despite being questioned by them twice. Without the DNA link he, too, would still be a free man. As we report today, the five suspected killers of Stephen Lawrence, whose racist murder in south London remains officially unsolved, may be brought to justice on the basis of new DNA evidence.

That the forensic use of DNA is saving lives and bringing killers to justice seems self-evident and has sparked a controversy over whether it should be extended to cover everyone in the country. Advocates of a universal database say that it would increase clear-up rates, particularly for assault, rape and murder. The detective who led the Bowman investigation said: “It is my opinion that a national DNA register could have identified Sally Anne’s murderer within 24 hours.” The innocent would have nothing to fear, while the guilty would be caught. It could even deter criminals.

Tempting as this sounds, it should be avoided. The success of DNA profiling in these cases comes hard on the heels of its failure in the Omagh bombing. There, “low copy number DNA” matching was used and rejected by the courts. Even conventional DNA evidence is not foolproof. The Home Office admits that in more than 50,000 instances since 2001, DNA samples taken from the crime scene resulted in multiple matches. This does not mean that the evidence is no use; most matches are quickly eliminated from police inquiries. But the problem would become more serious, and dangerous, if the database covered all 60m people in Britain.

There are other potential drawbacks. DNA can be left at a crime scene by people who were not there, so-called secondary transmission. It can result in ridiculous prosecutions, such as when a white man was recently brought to court on a rape charge when the victim had clearly identified her attacker as black. The police themselves argue for caution. Chief Constable Tony Lake, who speaks for the Association of Chief Police Officers, said that any extension of the database had to be “reasonable and proportionate”. It also has to be financially justifiable.

The existing database, which covers about 4.5m people, is expanding rapidly anyway. The latest figures for 2005-6 show that 715,000 people were added to it. A case in the European Court of Human Rights could mean that more than half a million records added since 1995 will have to be removed. The trend, however, is strongly upwards.

In the end, the argument against a comprehensive database is straightforward. The innocent may have nothing to fear but equally they also have no reason to hand over their genetic fingerprints to the authorities. Government is increasingly intrusive in our lives and reckless in its handling of our data. Putting everybody’s DNA on a national register would increase the potential for intrusion and for incompetent bureaucrats to lose our intimate details. And who knows how it might be used in the future? Last week's cases, however appalling, do not argue for a comprehensive DNA database.