The prime minister has an open mind on the issue of ID cards, Number 10 insisted on Monday.

The intervention came ahead of this week's publication of a consultation paper expected to back the introduction of a nationwide system of "entitlement cards".

"The prime minister's position is that we will await the outcome of the consultation," said a spokesman.

"The consultation document clearly sets out the pros and cons as we believe them to be so.

"In an exercise like this it is better for the prime minister to wait for the outcome before expressing an opinion."

But the plans are set to come under furious fire - with up to 100 MPs, pressure groups and unions pledging to wage a "war of attrition" against the Home Office.

The Tories have warned the home secretary that any proposals must distinguish between voluntary "entitlement cards" and compulsory "identity cards".

"I wouldn't want Britain to become the sort of place where you can't walk down the street without identity on you," said the shadow home secretary, Oliver Letwin.

Whilst the government claims that no final decision has been taken on ID cards, sources close to the home secretary have let it be known that he is sold on the idea of "entitlement cards".

Simon Hughes, the Lib Dem home affairs spokesman, said: "With Mr Blair in Downing Street and Mr Blunkett in the Home Office, Big Brother in the UK is in danger of becoming a reality.

"The government must guarantee that its new proposals will not become 'vouchers' by another name. Entitlement Cards must not be allowed to become another burden which falls only on the poorest in society."

MPs and civil rights groups are appalled at the proposal and have vowed to rebel against the government.

Peter Lilley, the former Conservative cabinet minister, has urged Blunkett to think again.

"The last government was offered the same idea by every new minister for IT as the remedy for all our ills from crime to shortage of kidney donors. As I pointed out, one normally starts with an illness and looks for a treatment," he says.

"Only quack medicines claim to cure ailments before they are diagnosed."

Lilley says the previous government examined plans for ID cards but concluded they were "a pretty dumb idea".

The home secretary believes that ID cards would assist in the war against terror, cut down on welfare fraud and help reduce crime.

He recently introduced ID cards for asylum seekers - a move which was criticised by civil rights groups.

Mark Littlewood, the director of campaigns at Liberty, says any attempt to extend the system to the rest of the country will prove counter-productive.

"There is no good evidence that ID cards would help tackle illegal immigration or domestic crime," he said.

"There is, however, ample evidence that compulsory ID cards - and the police powers that could accompany them - can do real damage to community relations."

Lilley, who was social security secretary until 1997, warns that attempts to introduce ID cards in Australia were quickly shelved in the face of widespread public opposition.

"A massive campaign was launched. Concern focused on its implications for privacy. Eventually the polls showed nearly 90 per cent against the proposal, and it fell in the Upper House. I look forward to Blunkett's plan suffering a similar fate," he says.