Identity cards would not have stopped the London bombings which killed more than 50 people and injured many more, Charles Clarke has said.
Clarke doubts any measures would have stopped the attacks
But the home secretary said on balance he believed ID cards would help rather than hinder the ability to deal with particular terrorist threats.
He also suggested that in future civil liberties might have to be curtailed.
Consideration would need to be given to checks on people boarding tube trains, ID cards and data exchange, he says.
He also insisted the small reduction in the official threat level made before the bombings had made no difference to the attacks.
Asked by BBC Radio 4's Today programme if ID cards could have prevented Thursday's atrocity, Mr Clarke said: "I doubt it would have made a difference.
"I've never argued ... that ID cards would prevent any particular act.
"The question on ID cards, but also on any other security measure actually, is on the balance of the ability to deal with particular threats and civil liberties, does a particular measure help or hinder it?
"I actually think ID cards do help rather than hinder.
"If you ask me whether ID cards or any other measure would have stopped yesterday, I can't identify any measure which would have just stopped it like that."
However, he later told the BBC that ID cards had helped officials identify the perpetrators of last year's bomb attacks on packed commuter trains in Madrid.
ID argument still stands
But Conservative shadow home secretary David Davis said the bombings had not changed his party's opposition to ID cards.
"The home secretary admitted himself that ID cards would not have prevented this and may not even have helped in the investigation afterwards so it doesn't change the argument," Mr Davis told BBC News.
The prime minister had said this was an attack on people's lives and their way of life.
Mr Davis added: "An important part of our way of life is maintenance of our civil freedoms."
In a separate interview on BBC Breakfast, Mr Clarke was asked about how people could be kept safe in the wake of the attacks.
"I don't think any of us want to live a life where we have to go through security checks every moment of our lives and so we have to find the right way to move forward," he said.
"The key thing is to study after these terrible events what needs to be done and discuss properly what alternative solutions there are."
Asked if that meant civil liberties being curtailed, Mr Clarke said: "I always hope not.
"But whether we look at identity cards or you look at exchange of data or you look at particular types of checking machinery when you get on a Tube train or whatever it might be, those are issues that have to be debated.
"Today isn't the time for those debates. Today is the time to track down the perpetrators and see that we can try and make the arrests that are necessary.
"People will have to look at the options and decide which is the right way to proceed."
Mr Clarke also said the decision recently to downgrade the official level of threat to the UK from severe general to substantial "wouldn't have changed events".
And the fact that 1,500 Metropolitan Police officers had been sent to Gleneagles to police the G8 summit had made no difference to the response.
"There were suggestions yesterday that the fact that we put resources into Scotland to deal with the G8 summit had lead to some lower position in London, but that is simply not true," he told Today.
"The London police, the Metropolitan Commissioner and all his colleagues at all levels will tell you that we had a complete ability to deal with the situation yesterday. I spoke to them directly about it."
Meanwhile, QinetiQ, the privatised former Defence Evaluation and Research Agency, and London Underground have rejected claims made in the Times newspaper that body scanners are to be used on the Tube.
The two organisations say the report is "inaccurate" and there are no plans to use the scanners. QinetiQ is providing some equipment but cannot discuss it.