The Government's plan to bring in identity cards has run into further problems after it emerged that the police would be able to use the national database to check fingerprints found at crime scenes.

Tony Blair was accused of contradicting previous assurances that the police would not be able to go on "fishing expeditions" when the scheme takes effect. He said ID cards would help to solve crimes, in an attempt to reassure 28,000 people who signed a petition against the cards on the Downing Street website.

In an email to the signatories, the Prime Minister argued that the scheme would help the police bring people guilty of serious crimes to justice. "They will be able, for example, to compare the fingerprints found at the scene of some 900,000 unsolved crimes against the information held on the register," he said.

Opposition parties claimed Mr Blair was "changing his tune" and that they never realised the police would be able to use the database in such a way.

Damian Green, a Tory home affairs spokesman, said Mr Blair's comments went "flatly" against the Government's undertakings to Parliament. "Obviously it has huge implications for people's privacy if the authorities are going to be allowed to go on a fishing expedition through the files of innocent people," he said.

Nick Clegg, the home affairs spokesman for the Liberal Democrats, said: "We were left clearly with the impression that the police wouldn't simply be able to go on fishing expeditions just with their own say-so.

"What is so distressing about this latest justification from the Prime Minister is that he has changed his tune almost week-by-week in justifying ID cards. First it was to do with terrorism, and he dropped that one. Then it was to do with benefit fraud, and he dropped that one. Now he's apparently pulled out of the hat this new justification."

Shami Chakrabarti, the director of the civil rights group Liberty, said: "The sheer grandiosity of the Prime Minister's ID card ambitions comes as little surprise. As public confidence in the Government's respect for our privacy wanes, the proposed intrusion grows and grows.

"It will take more than a friendly email from the big guy to repair this relationship of trust."

The Government insisted there was nothing new in Mr Blair's comments and that the police provision was set out explicitly in legislation passed by Parliament.

Joan Ryan, the Home Office minister, said police would have to check fingerprints against all their databases before requesting assistance from the Identity and Passport Service.

Ms Ryan added: "Surely no one would suggest that we should put obstacles in the way of police investigating crime."

A Downing Street spokesman said: "Anyone who says the Prime Minister has changed his tune on this either has a very short memory, or is choosing to ignore the facts."

He said that the draft ID Cards Bill in April 2004 made clear that the police would be able to check crime scene fingerprints against those held on the register.