Mr Blair looks on as an asylum seeker is finger-printed in Croydon

Human rights group Liberty described the micro-chipped passport trial as a "masquerade" to test out the technology needed to introduce ID cards.

The six-month study, unveiled by Home Secretary David Blunkett, puts fingerprints, iris or facial recognition data onto a microchip in the passport.

The Home Office said the technology could be incorporated into ID cards, but insisted it had no specific plans to stage a pilot ID scheme.

However Liberty said the passports hid Mr Blunkett's real objective.

Liberty spokesman Barry Hugill told BBC News Online: "We are reliably informed that ministers have been told they would need primary legislation if they wanted to test out a national identity card scheme.

For the government to say this isn't about ID cards is disingenuous Barry Hugill Liberty

"But they don't need legislation if the trial is for passports, even though it's the same technology that would be used in an ID card scheme.

"Mr Blunkett has said he wants the national identity card to be 'piggybacked' on the plastic passport and driving licence cards, so for the government to say this isn't about ID cards is disingenuous."

The UK Passport Service is hoping to begin implementing the new "smart" passports by April 2005.

Newspaper reports suggested the passport trial would take place this autumn in a small market town with a population of about 10,000 people, but the Home Office said it would not necessarily be structured along these lines.

A spokesman said the venue and date of the trial could not be confirmed until the project details had been finalised.

Asylum seekers

Mr Blunkett also announced on Wednesday that biometric chip technology is to be used on visas for entering the UK, to try to stop people entering the country illegally.

This scheme is already being piloted for those applying for a visa in Sri Lanka, who must provide a record of their fingerprints.

Asylum-seekers applying at the Asylum Screening Unit in Croydon are also already fingerprinted and issued with a micro-chipped ID card.

Mr Blunkett was joined by Prime Minister Tony Blair on a visit to see the technology being used at the unit, on Wednesday.

The Home Secretary said the new visas would provide a more secure way of confirming someone's identity, even if they had destroyed their documents.

The government says it has no plans for an ID card scheme

"It will also make it easier to discover if someone is in this country illegally by over-staying their visa period and to remove those who do not have a right to be in this country," he said.

The Home Office did not give details of how widely the technology would be used or how quickly it would be introduced on visas.

Liberty claim a national ID card scheme would cost at least �3bn to set up and would be a waste of resources.

Mr Hugill said an ID scheme in France had increased tension between police and ethnic minority groups, who said they were regularly being stopped and asked for their ID cards.

Home Office minister Beverley Hughes recently confirmed that over 5,000 of 7,000 responses to a government consultation on a national ID card scheme were against the idea.