Tony Blair praised West Midlands Police for their success using DNA technology to solve "cold cases".

The Prime Minister highlighted the work of the force as he urged police across the country to follow their lead.

Mr Blair was speaking in Downing Street as he announced the second phase of a Home Office scheme to re-open case files going back as far as 20 years.

Advances in DNA technology now mean old stains from crime scenes can be reanalysed using new techniques and matched against the profiles on the database - known as "cold case" reviews.

This enables the reopening of cases not because of new evidence but because of new DNA forensic techniques.

With each advance, the probability of obtaining a positive result in a cold case increases.

The Home Office project - Operation Advance - has so far achieved 21 convictions, with several other suspects arrested and awaiting trial or sentence. Sentences passed so far total more than 100 years, plus three life terms.

A second phase of the project has now been launched, bringing Home Office spending on Operation Advance to £691,000.

Mr Blair said: "West Midlands Police are to be congratulated for pioneering new scientific advances with DNA to re-open and solve horrendous rape and sexual assault cases.

"Some of these cases go back 20 years, but by setting up a special cold cases team with support from the Home office, West Midlands Police are bringing criminals to book."

Since changes introduced by the Government in 2001 and 2003, there are now 3.6 million DNA records held on the database, including Mr Blair's own profile and those of 18,000 other volunteers.

According to Government sources, the majority of the active criminal population are now thought to have their DNA recorded. Police now receive more than 3,500 DNA matches a month - more than double the figure in 1998/99.

The 45,000 crimes for which DNA matches were achieved in 2005/06 included 422 homicides, 645 rapes, 256 other sex offences, 1,974 other violent crimes and more than 9,000 domestic burglary offences.