Opposition parties have expressed anger that all fingerprints collected for ID cards will be cross-checked against prints from 900,000 unsolved crimes.
The government plans a national identity register
No 10 insists it was always the plan to allow the checks, but the Tories and Lib Dems say they were not aware of it.
In an e-mail to 27,000 signatories to an anti-ID card petition, Tony Blair said the cards would help bring "those guilty of serious crimes" to justice.
The Conservatives and the Lib Dems both oppose plans for identity cards.
Mr Blair was responding to the people who signed the e-petition on the Downing Street website.
The petition stated: "We the undersigned petition the prime minister to scrap the proposed introduction of ID cards. The introduction of ID cards will not prevent terrorism or crime, as is claimed. It will be yet another indirect tax on all law-abiding citizens of the UK."
Mr Blair outlined the benefits that the government and security services believe will come from having ID cards - and the register of biometric details - to combat crime and terrorism.
He wrote: "I believe that the National Identity Register will help police bring those 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."
The Home Office said the police would not be allowed to search the register, saying its Identity and Passport Agency would do any database searching.
The PM's official spokesman said that it was always the government's intention that the police could get access to fingerprint data contained on the planned register.
Under the Identity Card Bill, first published in April 2004, approved authorities would be allowed access to "limited parts" of people's details on the register, with the person's consent, so they could check somebody's identity.
The bill also said that details could also be given without consent to police, intelligence agencies, customs and tax authorities and certain government departments for preventing and detecting serious crime, ensuring national security, investigating benefits fraud and protecting Britain's "economic well-being".
Mr Blair's spokesman said: "If the police ask for fingerprints to be cross-checked, that has always been part of the intention of the bill."
The Conservatives and the Liberal Democrats are opposed to the ID scheme, saying it will not prevent illegal immigration or identity fraud and will cost far more than government estimates.
The ID card scheme approved by Parliament means everyone wanting a new passport will need to provide details - such as fingerprints - from next year.
They do not have to get an identity card however, although the government says it intends to bring in a bill if it wins the next election to make identity cards compulsory.
The Police Superintendents Association was pleased with Mr Blair's statement.
Its president, Rick Naylor, said: "The public would want us to do that because it is important to them, particularly the victims of crimes that the crimes are solved.
"We have many, many marks on record that we don't have an owner for. Anything that would help us solve crime would certainly reassure the public."
Lib Dem home affairs spokesman Nick Clegg told BBC Radio 4's World at One: "We were left clearly with the impression that the police simply wouldn't 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's changed his tune almost week by week in justifying ID cards.
"First it was to do with terrorism, then he dropped that one. Then it was to do with benefit fraud, then he dropped that one.
"The public will rightly feel extremely confused if the government can't make it's own mind why it wants to spend billions of taxpayers money on something which they can't be consistent about."
He said there had been a lack of debate about whether a "surveillance state" was wanted by the public.
For the Conservatives, shadow home office minister Damian Green said: "It flatly goes against all the undertakings the government gave Parliament during the course of the bill.
"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.
"Everyone assumes that fingerprint technology is 100% accurate. And it just isn't, experience tells us that it's not infallible.
"With the vast number of crimes involved, it is guaranteed there are going to be miscarriages of justice if the government goes down this route."
But the Home Office minister responsible for the identity card scheme, Joan Ryan, told the same programme that any check of the identity register would be made by approved Identity and Passport Service staff.
"There won't be any fishing expeditions. That is complete nonsense. That is not what can happen. We've always said one of the real advantages of identity cards would be the fight against crime and protecting the public.
"If 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?"