Charles Kennedy has challenged the government to explain how identity cards will make a difference in the war against terrorism.

Ahead of Thursday's Commons debate on the bill to introduce the system, the Liberal Democrat leader said Tony Blair was wrong to claim it would be an effective means of preventing attacks.

Speaking during prime minister's questions on Wednesday he said Blair had claimed ID cards would be "essential" to intercepting groups such as al Qaeda.

"Given that ID cards did not prevent the Madrid bombings, the 9/11 bombers travelled under their own identities, it is not going to be compulsory for 10 years and visitors to this country won't require them - how, then, is it going to make such a difference to the fight against terrorism?" he asked.

Blair said the bill would enable biometric data to be used in passports and the cards, which would give the security services more help in tracking terror suspects.

"We can construct an identity card that gives us the best possible protection against crime and against terrorism," he said.

"There is no system that is going to prevent all crime and all terrorism.

"The question is does it enhance the security of this country.

"And when this technology is available and we are going to be applying it, in any event, to visas and passports, it seems to me to make sense to use that technology, to give us an identity card and bring us into line with best practice around the world."

Kennedy replied that "millions of pensioners don't have passports" and that his party had consistently opposed the idea.

"That is another reason why we, as we have done before, will be voting against the legislation," he said.

But Blair insisted that there was no civil liberties restriction in the bill, which the Lib Dems claim.

"I don't think it is a breach of anyone's identity, most people already carry some form of identity on them," the prime minister said.

"I think this is long overdue and we should get on and do it."