The introduction of identity cards to protect the UK against crime and terrorism "is long overdue", Tony Blair told MPs at Commons question time.
Identity cards are proving controversial
"I don't think it is wrong or a breach of anyone's civil liberties," he said.
But Lib Dem leader Charles Kennedy questioned the plan when 10 million people, including many pensioners, did not even have a passport.
The Conservatives have so far backed the idea of ID cards in principle but are unhappy with details of the scheme.
Controversy over the plans is likely to come to a head on Thursday with MPs debating the remaining stages of the Identity Cards Bill.
While government and Tory front benches support the measures, which will introduce a single, universal ID Card for all UK nationals, they are opposed by the Liberal Democrats and a number of MPs on both sides.
During the weekly prime minister's question time Mr Kennedy pressed Mr Blair to explain why he believed they were essential in the fight against terrorism when they had failed to prevent the Madrid bombings and 9/11 attack.
The prime minister said the police and security forces backed the scheme, particularly when it is launched alongside biometric passports.
"When this technology is available, when 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 and give us an identity card and bring us into line with best practise around the world," he said.
But Mr Kennedy said the plans would mean 10m people who do not have passports, or any intention of getting one, would have to undergo tests and pay "for the privilege" of having one.
He said this was "a further reason" why he and his colleagues would be opposing the Identity Card Bill on Thursday.
Mr Blair insisted that there would be a rigorous evaluation and a debate before compulsory ID cards were introduced.
"I believe that many people now recognise we can genuinely make a difference to our own security, the fight against crime, the protection of our public services," he said.
"When organised crime and terrorism is far more sophisticated than ever before, I don't think it is wrong or a breach of anyone's civil liberties to say we should have an identity card.
"Most people carry some form of ID anyway - I think it is long overdue and we should get on with it."