Hundreds of murderers, rapists and other serious criminals may have escaped prosecution because forensic scientists failed to test properly for DNA evidence for five years. More than 2,000 cases are being urgently reviewed — including that of Rachel Nickell, the former model murdered on Wimbledon Common — after the Forensic Science Service admitted that it might have missed crucial evidence.

The service, which carries out most DNA tests for police forces in England and Wales, expects to recover DNA in at least 200 cases where it had said that none existed.

The Times has learnt that chief constables believe that the total could be higher. The failure involves the processing between 2000 and 2005 of “low-copy-number” tests, used to identify microscopic quantities of DNA.

The Association of Chief Police Officers wrote to all police forces yesterday asking them to carry out a review of their serious crime cases.

This is likely to include all unsolved serious crimes since 2000 where no DNA samples were found. It could include the murder of Margaret Muller, 27, an American artist, who was stabbed after going jogging in Hackney, East London, in February 2003.

Tony Lake, the association’s expert on the use of forensic science, admitted that there was a huge amount of work to be done to find out how many cases could have been affected. Mr Lake, the Chief Constable of Lincolnshire, added: “Are we anxious? Are we concerned? Yes, of course we are. We have to establish whether there are offenders out there who could have been caught previously.”

The blunder relates to cases where the Forensic Science Service analysed tiny samples of blood or saliva for a DNA profile. As a result of advances in testing techniques, it was possible, from 2000, to find this DNA evidence in very small traces of body fluids.

The service was apparently using this low-copy-number technique, but for five years was applying it in a different way. This meant that its scientists failed to locate evidence that private forensic laboratories could have picked up.

The failure is the most damaging mistake in the history of the service.

Problems with the tests were identified last summer when Scotland Yard reviewed evidence on the murder of Ms Nickell, who was stabbed 49 times and sexually assaulted in front of her son in July 1992. The Forensic Science Service said that it had not found any positive results when it carried out the low- copy-number tests in 2001 on material from the scene. But in a review of the case last year a private company discovered DNA. Detectives may now charge a convicted sex killer with the murder.

John Reid, the Home Secretary, has demanded a report from the service by the end of the week. David Davis, the Shadow Home Secretary, said that it was an incredibly serious matter. “It opens up the prospect for a large number of miscarriages of justice,” he said.

The Forensic Science Service said that it gave Scotland Yard a report on the problems last October, which was passed to the association. Home Office ministers were alerted and the association sent a senior forensic scientist to tour every laboratory in the country to check working practices. A conman used bogus qualifications bought on the internet to pose as an expert forensic scientist in hundreds of civil and criminal cases including rape, armed robbery and death by dangerous driving. Gene Morri-son, 48, a Jamaican-born father of nine, was found guilty of perverting the course of justice, perjury and obtaining money by deception by Manchester Minshull Street Crown Court. He will be sentenced today.

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