Downing Street last night defended itself against a fresh attack on its flagship ID cards policy after Tony Blair said the police would be able to use the national database to check against fingerprints found at crime scenes.

Opposition parties claimed that once again the Prime Minister was expanding the policy to enable police to go on "fishing expeditions" through the details of innocent people and infringe their civil liberties.

No 10 hit back, insisting the ability of police to cross-check ID cards against fingerprints from 900,000 unsolved crimes had always been the plan. But the Liberal Democrats and Conservatives, who strongly oppose ID cards, claimed they were unaware of it.

Nick Clegg, LibDem home affairs spokesman, 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, apparently he's pulled out of the hat this new justification."

Mr Blair explained government policy in an e-mail to 27,000 signatories to an anti-ID card e-petition, claiming ID cards would help bring "those guilty of serious crimes" to justice. He said the national identity register would be helpful to police. "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."

However, Damian Green for the Tories said these comments went "flatly" against the government's previous 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."

Shami Chakrabarti, director of Liberty, the civil rights group, added: "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 e-mail from the big guy to repair this relationship of trust."

However, Joan Ryan, Home Office minister, rejected any suggestion of a "fishing expedition" by the police. She made clear officers would have to check fingerprints against all their databases before requesting assistance from the Identity and Passport Service.

"They can approach IPS and approved IPS staff will be able to search the national identity register to see if we can achieve a match for that fingerprint. So there won't be any fishing expeditions. That's complete nonsense, it's not what can happen."

Ms Ryan stressed that the Prime Minister was merely repeating the government's position on ID cards. "We always said one of the real advantages of identity cards would be the fight against crime and protecting the public. As I said, if the police want to check fingerprints found at the scene of the crime that they can't find on their own databases, then they will work with IPS staff.

"And surely no-one would suggest that we should put obstacles in the way of police investigating crime and bringing offenders to justice."

Later, a Downing Street spokesman also defended Mr Blair, saying: "We have been utterly consistent in what we've said about this. One of the core reasons for the ID Cards Act is to help detect and deter crime."