Plans for a national ID card scheme have been branded "farcical" after suggestions it might misidentify people with brown eyes or men who go bald.
A stand-alone ID card will cost £30, ministers say
The Home Office's Tony McNulty admitted some technological "difficulties" with some of the biometric checks.
That was why the cards would employ 13 such features in all, he said.
Tory spokesman Edward Garnier called the scheme an "expensive waste of time". Lib Dem Mark Oaten said ID card plans were "descending into farce".
The Commons will vote on Tuesday whether to give the Identity Cards Bill its third reading, with some Labour MPs expected to oppose the measure.
Tory home affairs spokesman Mr Garnier said: "Week by week the government has had to change their case on ID cards as more and more defects are revealed.
"The brown eyed problem is just the latest and it yet again shows that this ID card proposal is an expensive waste of time which will achieve nothing save a gross infringement of people's civil liberty."
Liberal Democrat home affairs spokesman Mr Oaten said: "What kind of government spends billions of pounds on a piece of plastic that might get your identity wrong if you're bald or have brown eyes?
"The ID Card Bill is rapidly descending into farce but the costs are still rising. Biometric failure rates are still far too high for anyone to have confidence in this scheme."
A newspaper report has suggested one in 1,000 people could be identified incorrectly by scans.
Mr McNulty said on Sunday: "There are difficulties with the technology, not least in terms of people who have difficulties with their eyes anyway, not least with people with brown eyes rather than other coloured eyes, and all those are being factored into the equation.
"None of these problems are new, but increasingly as biometrics are more and more used... we think the technology can only get better and better and better."
The government is planning to use face, iris and fingerprint scans to identify people.
But studies have found that being scanned in the wrong type of light or in shadow could lead to an inaccurate identification.
The Independent on Sunday reported that internal studies for the government had warned that manual labourers, typists and pianists could wear down their fingerprints, affecting readings.
Men who go bald also risked being identified as someone else.
The paper said trials had demonstrated that the biometrics of black, elderly and disabled people had a higher chance of being incorrectly matched.
Last week, Home Secretary Charles Clarke revealed people would have to pay £30 for a stand-alone identity card.
But it is expected most people will want a combined passport and ID card, costing an estimated £93 each to make. The cards will be valid for 10 years.