Serious doubts are being raised about a new secure identity system being incorporated into new UK passports from the end of 2005.
From next October, UK passports will include face scans
Biometric facial recognition will be brought in as part of an international agreement to target terror and fraud.
But trials suggest the technology has a 10% failure rate, the BBC has learned.
Some experts say the technology could be unsuitable for the high volume of travellers it will deal with, and may fail to improve airport security.
Facial biometric details will be stored on a microchip which will be carried on all new UK passports from October 2005.
This will then be compared to a biometric scan taken of travellers' faces at certain airport check-ins.
Britain has agreed to introduced the biometrics ahead of a deadline to avoid UK citizens needing visas to visit the US when new regulations come into force there next year.
The BBC's Rory MacLean says unpublished studies carried out in Europe and the UK found the computerised scans failed in about one in 10 cases.
The problems are apparently due to the technology's sensitivity to light conditions.
Professor Angela Sasse of University College London - who has made a study of biometrics, said she was very doubtful whether facial scans were a practical security measure yet.
"It will be a huge problem if facial biometrics cannot always correctly identify genuine passport holders," she told the BBC.
The UK Home Office said information stored on a passport would be very different to biometrics being considered for national ID cards that Home Secretary David Blunkett is keen to see introduced.
A spokesman said a proposed ID card would include "facial, iris and fingerprint" biometrics.
"This will make it much easier to deal with any failures that might occur," he added.
Labour's John Denham, chairman of the Home Affairs Select Committee, said: "The government has got to do two things. It has got to assure us it getting independent advice.
"They must also say what level of security they want the system to provide at each level."
But Home Office Minister Fiona Mactaggart said the public should not be concerned.
"This technology is not foolproof. No country is looking just to depend on the biometrics technology. They are relying on all the other things that are used."
She said immigration officials will also be allowed to question people at airports just as they do now.