Iris-scanning machines are not reliable enough to be used at airports and will cause long delays for passengers, a Conservative MP has claimed.
ID cards will use biometric details, with iris scans a possibility
Ben Wallace said an official report showed the scanners had so far failed half of their assessments.
This was further proof that government plans to introduce biometric ID cards were "running off the tracks", he told BBC Radio 4's Today programme.
But the Home Office said initial problems had been dealt with.
The system is now being tested at Heathrow, Gatwick and Manchester airports.
The technology works by photographing a passenger's iris patterns and matching them to their passport details. These will be stored on a microchip.
This allows customers to skip passport checks and removes the need to see an immigration officer.
The Home Office spokeswoman said the iris-recognition system remained "an option" for ID cards, which will be introduced from 2009.
Tories and Liberal Democrats oppose the scheme.
Mr Wallace told the BBC: "The pilot failed half its assessments: it wasn't available when it was needed at the right level; when the system crashed, it took over eight hours to fix.
"Iris recognition is one of the main planks of the biometrics the government said made their ID scheme foolproof.
"In an answer to me, the minister said that iris recognition had been chosen as a biometric because it outperformed all others. In this case, it doesn't."
He added: "I would say that this scheme is certainly running off the tracks and, at worst, is becoming unworkable."
Passengers would pay the price for the lack of "proper rigorous testing", he said.
Mr Wallace added: "Millions of people are going to arrive at Heathrow and wait in a queue because the system has crashed for over eight hours."
Liberal Democrat home affairs spokesman Nick Clegg said: "Yet again the government has tried to bury another piece of bad news about its doomed identity cards project.
"With each successive announcement it becomes more obvious that the technology simply isn't good enough to sustain such an expensive, illiberal and unnecessary scheme."
The Home Office spokeswoman said the government remained committed to the use of biometrics for ID cards, with iris scans an option.
She said there had been problems but said the point of conducting trials was to identify such issues.
More than 50,000 people have signed up to trial schemes, with 150,000 passengers using through the new-style security checks last year, the spokeswoman said.