Liberal Democrat leader Charles Kennedy has called on opposition parties to "make a stand" against government plans to introduce identity cards.
The case for them was "unpersuasive", he told BBC Radio 4's Today programme.
The ID cards bill faces a rough ride in the Commons after a minister admitted there were technical "difficulties".
Ministers say cards, holding biometric data, are vital to fight terrorism. But shadow home secretary David Davis says they will undermine civil liberties.
Mr Kennedy said the home secretary, Charles Clarke, had made clear after the July bombings in London that ID cards would not have prevented the outrages.
Gathering the necessary biometric details from everyone in the UK would cause "mass inconvenience to a lot of people", particularly the elderly, the disabled and those living in rural areas.
Home Office Minister Tony McNulty has admitted people might be wrongly identified by one of the biometric tests to be used on the cards.
He told Today: "I think people do understand more and more their security, their identity is under threat and we need to resolve that.
"What's not going to change is increasing use of technology in a whole host of areas in terms of our daily lives, whether we like it or not.
'Plastic poll tax'
"And I think the security and sanctity of the individual identity is important and we think with all the contingencies we've built in, that this the way to go forward."
David Davis called the cards, which the government says, will cost £30 each, a "plastic poll tax".
He added that they would "do nothing to combat benefit fraud, illegal immigration or terrorism. The only thing we can be certain of is that they will undermine our civil liberties."
Some Labour MPs are expected to oppose the bill when it returns to the Commons for its remaining stages debate.
Tory and Liberal Democrat MPs are also expected to vote against the measure.
During the Second Reading debate in June, 20 Labour MPs voted against the bill, including four of Tony Blair's former ministers.
Ministers are planning to use face, iris and fingerprint scans to identify people.
Labour MP Neil Gerrard has tabled an amendment to the bill that would make it possible for people to apply for a passport without having to submit their details for the ID cards database.