Tony Blair has said that ID cards will not be scrapped although the detailed implementation "may change".

Speaking in the Commons, the prime minister said the scheme remained vital for national security.

His comments came as Conservative leader David Cameron cited leaked emails from Home Office officials and warned the scheme was becoming a "botched job".

Blair said it "isn't correct" to say that ID cards are in trouble.

"It is important we proceed with identity cards for the very simple reason that if we don't have a proper identity card system we will not be able to track illegal immigrants in this country, prevent identity fraud and abuse, and it is for that reason we most certainly will proceed to introduce identity cards," he said.

Cameron said officials thought the scheme might not be introduced in full before 2026.

Blair said the view was that the details for implementation "have to be right".

"It a huge programme and there are bound to be changes along the way," he said.

"But the basic point of introducing identity cards, alongside the fact that we will have to have biometric passports introduced in any event, is of essential importance to the security of this country."

The prime minister also predicted the Tories would "end up agreeing with this proposal.... because it is right and necessary for the country's security".

"We will pursue identity cards because they are right," he insisted.

Earlier, Downing Street pointed out that the home secretary had intervened in a Commons debate on Tuesday to deny reports that the government was "backing away" from ID cards.

The prime minister's official spokesman said John Reid was firmly of the view that ID cards would be a useful tool to tackle problems including immigration and terrorism.

"The more he sees... the more he believes that ID cards are a crucial part of the answers to those questions," said the spokesman.

However, Number 10 said that there "may be a slight resequencing so that we address the issue of foreign nationals first".

But the spokesman insisted that "the timetable is broadly on track".