Identity cards might not become compulsory for all Britons, Gordon Brown has appeared to suggest.
Identity cards are opposed by the Tories and Lib Dems
Anyone getting a passport from 2010 will have to get a card, and ministers had said they would be compulsory for all if Labour won the next election.
But, in an apparent softening of that line, Mr Brown described compulsion only as an "option" which is "open".
The Tories and Lib Dems oppose the scheme and say they would axe identity cards if they got into power.
The parties, who oppose the cards on cost, effectiveness and civil liberty grounds, have said they would seek to make them a key issue at the next election.
The current scheme will see anyone applying for a passport having to give their biometric details for a national identity register, although it will be possible to opt-out of getting a card until January 2010.
The decision not to make getting ID cards compulsory immediately for all, came as a result of Parliamentary opposition to the scheme.
Instead, in February 2006, MPs voted to back a government compromise requiring new legislation before it becomes compulsory for all citizens to get an ID card.
The then prime minister Tony Blair said the government had "won the argument" on ID cards and legislation introducing them would be a "major plank" of Labour's next general election manifesto.
And the Home Office's website says that the National Identity Scheme "will eventually become compulsory... this means that all UK residents over 16 will need to have an ID card".
But, asked at his monthly Downing Street media conference if ID cards had to be compulsory for all citizens in order to be effective, he replied: "That's the option we have left ourselves open to but we haven't legislated for it."
Mr Brown said he believed the government would be able to persuade British citizens of the benefits of identity cards.
"Over the course of the next few months, people will see there is some wisdom in the argument we have put forward for identity cards themselves.
"If you look at the information that we are asking people to give for their identity card, it is not much more than they is actually required for a passport
"But the advantage people have from an identity card is that that information cannot be used without biometric identification.
"And that's why are starting with foreign nationals and that's why we will move further, linking, if you like, passport information over the course of the next few years.
"But we leave open a Parliamentary vote on the decision about compulsion."
Former Home Secretary David Blunkett, who introduced the initial identity card scheme, said Mr Brown's words were "in line" with the compromise struck with MPs.
But he added: "In my opinion, without it being mandatory, there is little point in doing it."
Phil Booth, of anti-ID card campaign group No2ID, agreed that the scheme could not operate as the government intended without being compulsory.
He said there needed to be a "fundamental U-turn" and said the comments might have been a case of Mr Brown softening his language to appease Labour MPs who were against the cards' introduction.
"He has got no option politically but to soften the tone."