The government is facing another Lords defeat this afternoon on ID cards as opposition peers accused it of introducing compulsory biometric cards by "stealth".
Tory and Liberal Democrat peers are preparing to vote against provisions in the identity cards bill, passed by MPs last month, which require all new passport applicants to supply biometric details onto the national identity register.
The Tory leader in the Lords, Lord Strathclyde, said this effectively made ID cards compulsory for the vast majority, in breach of Labour's manifesto promise that they would initially be voluntary.
Lord Strathclyde told BBC Radio 4's Today programme: "Under this law, whenever you apply for or wish to renew your passport, you will at the same time be obliged to fill in the documents to have an ID card.
"That fundamentally changes what the government said they were trying to do, which was to have a voluntary ID pass system.
"We want to go back to an entirely voluntary scheme. The reason why this is so important is that if we don't get it right, some 85% of the people of this country will be affected within the next five to six years."
Lord Strathclyde dismissed the government's argument that peers were breaking a convention known as the Salisbury Doctrine whereby the upper house does not vote down manifesto commitments.
"That is why this is important, because what we are doing this afternoon is obliging the government to stick by its manifesto, not oppose it," the peer said. But home affairs minister Andy Burnham told the programme: "The manifesto says very clearly that we would seek to introduce ID cards incrementally as people renew their passports. It couldn't really have been clearer.
"There will be no compulsion initially for people to have an ID card. There is a good reason for doing it as we are, which is that the passport is changing. We will be introducing biometric information over the next couple of years. We wanted to do it in a managed and incremental way, but also we don't want there to be two systems," Mr Burnham said.
He added: "The effect of the amendment they are seeking to push today is that all the same information will be taken anyway, because we are getting a biometric passport, but we would lose the safeguards and the scrutiny and the legal framework that comes with the ID register."
Earlier, the Home Office announced that the first British biometric e-passports will be issued to applicants this week.
The new passports have added security features including a chip holding the holder's facial details in a bid to combat fraud and forgery.
The e-passports will be introduced gradually throughout 2006 and will be issued to all applicants by the end of August. The electronic chip will contain the digitally coded measurements of the holder's features, like the distances between eyes, nose, mouth and ears.
The information will be taken from the applicant's passport photo and can be used to identify them. The new document will also feature pages with intricate designs and new complex watermarks.
These technological advances are being adopted across the world to improve the security of travel documents and border controls, with more than 40 countries in the process of introducing e-passports.
Countries such as the UK, which are part of the US visa waiver scheme, must comply with new International Civil Aviation Organisation standards and begin issuing biometric passports, incorporating a facial image, by October to remain in the scheme.
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