The iris recognition technology that the Government plans to use on its new national ID cards has a 7 per cent failure rate, it emerged today.

Watery eyes and long eyelashes make it difficult for the biometric technology to match unique patterns on the iris of the eye.

The glitches emerged as a group of MPs from the Home Affairs Select Committee visited the UK Passport Service headquarters in London where the technology is being tried out.

Staff operating the cameras were unable to scan the iris of one of the committee members, Bob Russell, who suffers from an eye complaint.

Mr Russell said that he found the scan uncomfortable and it made his eyes water.

"I think this is going to cause serious problems for people who suffer with bright lights and people with epilepsy," Mr Russell said afterwards.

"I think it will be necessary at every machine to have at least one member of staff who is a qualified first aider to a high level. I can see people keeling over with epileptic fits."

The project director Roland Sables admitted to John Denham, the Home Affairs Select Committee chairman, that staff were on a learning curve and so far during the trials the equipment had failed to recognise matches in 4 per cent of cases.

"The pundits tell us that we should expect seven per cent across the board to fail with iris recognition, mainly due to positioning in front of the camera," added Mr Sables.

"Others are due to eye malformations, watery eyes and long eyelashes in a small percentage."

Some experts believed that the iris scanners may not work on those wearing hard contact lenses, said Mr Sables. This would be one of the factors analysed by the Home Office pilot.

Mr Sables said the ID card system, which also records fingerprints, may also be unable to register some people with faint fingerprints.

Manual labourers, particularly those who work with cement, were difficult to fingerprint, as were those who shuffle paper regularly. This could be overcome by the next generation of fingerprinting technology which reads bloodflow beneath the skin, he added.

Mr Denham said: "The registration process overall seems very simple. It will be interesting to see how simple it is for those with physical disabilities or difficulty adjusting their position in front of the iris camera.

"I would not be surprised if we need to have a more mobile camera rather then expecting people to shift their position."

He added: "Some of the crucial issues about the technology will be better informed at the end of the trial."

Biometric details, which may also include a facial recognition scan, are due to be included in the national ID cards.

David Blunkett, the Home Secretary, says that by 2013 he expects 80 per cent of the population to have a biometric passport or driving licence. At this point the government will decide whether to make the ID cards compulsory.

It would be the first compulsory ID card in Britain since the Second World War.

The pilot scheme is due to expand to Newcastle tomorrow, followed by Leicester and Glasgow later this month. Next month a mobile registration unit will begin visiting towns and cities including Belfast, Peterborough, Macclesfield, Liverpool, Harrogate, Middlesbrough, Sheffield, Birmingham, Torbay, Torquay and Bournemouth.

The Passport Service hopes to have registered 10,000 volunteers for its trial scheme by the end of August.