Hundreds of criminal cases involving murderers and rapists where the offender has gone free will be reviewed by forensic experts because crucial DNA evidence may have been missed.

Chief Constables of every force in England and Wales have been written to by the Association of Chief Police Officers and the Home Office warning them of the possible errors involving cases 2,000 cases tested between 2000 and 2005.

The problem surrounds DNA tests that were carried out by the Government owned Forensic Science Service involving Low Copy Number (LCN) - a sample containing very few cells.

Police and scientific advisers believe they were not properly using a special technique when testing for the microscopic traces of DNA on exhibits. As a result, vital evidence could not have been found.

The problem came to light last year during a review of the murder of Rachel Nickell, who was stabbed 49 times and sexually assaulted in front of her young son on Wimbledon Common in July 1992.

It is believed that in 2001 forensic teams missed small samples of DNA taken from her body that could have been linked to a convicted murderer currently detained at Broadmoor.

His DNA profile was on the national database at the time of the error, meaning it could have been matched to the DNA taken from Ms Nickell.

An ACPO spokesperson said: “The FSS were using an LCN technique in a different way to other forensic suppliers so basically they may not have obtained DNA profiles in cases other suppliers may have done.”

A joint statement between the Home Office and ACPO added: “Towards the end of 2006 we become aware that a small percentage of DNA samples may need to be re-examined as a result of differences in the way forensic suppliers were using new techniques to analyse forensic material between 2000 and 2005.”

They said that they kept the investigation secret on police advice, for operational reasons.

“As soon as Ministers became aware of this issue they asked ACPO to undertake an operational review to secure the nature and extent of it and to take remedial action,” they added.

A spokeswoman for the FSS said that a change in the way they tested LCN was discovered in late 2005 that allowed them “to understand how to increase the chances of getting a match and improve results.”

She said that of the 2,000 cases being retested they thought they may be able to get a result on at least 200 of them “because we are doing things differently”.

There has been claims that there has been an attempt to cover up the latest controversy to embroil the beleaguered department.

David Davis, the Shadow Home Secretary, said that it was an “incredibly serious” matter that meant there could have been a large number of miscarriages of justice.

Dangerous criminals may have escaped justice, he warned.

“It is another flaw in our criminal justice system that will leave dangerous people at large and put a lot of people at risk,” he told Sky News.

“It opens up the prospect for a large number of miscarriages of justice for reasons that are squarely at the door or the Home Office. The public must be getting tired of the serial failures at the Home Office.”

Mr Davis said that the Home Office knew about the problem last year and it now appeared as though it had tried to mount a “cover-up”.

The Home Office/ACPO statement added: “ACPO is very close to completing that work and has found no evidence that we should be concerned about standards being used today.

We cannot speculate on the numbers involved until the full review is complete.”

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