Long eyelashes and watery eyes could thwart iris scanning technology used for the government's ID card trial.
Thousands of people are needed for the ID card trial
An MP who volunteered to take part in the trial at the UK Passport Service headquarters in London complained the scanning was uncomfortable.
Home Affairs Select Committee member Bob Russell, who suffers from an eye complaint, said his eyes watered and staff were unable to scan his iris.
A project spokesman said the aim of the pilot was to iron out such problems.
The government is seeking 10,000 volunteers for the trial of the cards that include iris patterns and fingerprints.
"The pundits tell us that we should expect 7% across the board to fail with iris recognition, mainly due to positioning in front of the camera," project director Roland Sables told MPs.
"Others are due to eye malformations, watery eyes and long eyelashes in a
Hard contact lenses could also prove problematic, said Mr Sables.
So far in the trial, about 4% of iris scans have been unsuccessful, he added.
Mr Russell expressed concern about the scanning after his experience.
"I think this is going to cause serious problems for people who suffer with bright lights and people with epilepsy.
"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."
People with faint fingerprints would also be unable to register on the system, as would manual labourers, particularly those who work with cement or shuffle paper regularly, Mr Sables told the MPs.
Biometric details, which may also include a facial recognition scan, are due
to be included in passports from 2007.
By 2013, 80% of the population are expected to have a biometric passport or
driving licence, at which point the government will decide whether to make the
ID cards compulsory.
The pilot scheme is due to begin in Newcastle on Friday, 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.