Mismatched or unclear fingerprints could hamper the government's £5.6bn ID card scheme, independent experts warn.

The Biometric Assurance Group (BAG) says officials may struggle to cope with the number of false matches, which could run into tens of thousands.

Getting clear prints from the over-75s may also prove difficult, it adds.

The government says it is carrying out research into the issues raised - but it rejects BAG's call for iris scans to be used as a back-up to fingerprints.

When the ID scheme was first proposed iris scans were expected to be part of the biometric data stored on the cards.

But the government has now rejected this on cost grounds.

False matches

Everyone applying for a passport from 2010/11 will have to submit to a digital fingerprint scan, with the prints to be stored on a database.

They will then have a choice of a passport or ID card which the government says will help them to prove their identity when challenged by the police, border officials or in some commercial transactions such as with banks.

Any false matches - which could result in the wrong person being arrested or prevented from entering the country - will be dealt with manually.

In its annual report for 2007, published this week, the BAG suggests the government has underestimated how much time and effort this will take.

It says dealing with mistakes - called "exceptional handling" - will be a "large part" of the National Identity Scheme's work.

"Exceptional handling has a large impact not only on the technical elements of the scheme but on business processes, schedules and costs," the report says.

It also raises concerns about getting clear fingerprints from the four million over 75s and other people with "challenging biometrics" such as "mute, non-English speaking, blind or visually impaired" people.

Iris scans

In its response, the National Identity Scheme (NIS) said it had funded trials "which investigate the enrolment of people with characteristics that may make biometric enrolment challenging".

It said an enlarged fingerprint bureau was also being planned to ensure mistakes can be corrected quickly.

But the Identity and Passport Service (IPS) rejected a call for iris scan technology - potentially more accurate than fingerprints - to be developed as a fall-back.

"IPS accepts that iris biometric technology has potential but is not inclined to mandate its testing during the current procurement as it is unlikely to be used for scheme launch or immediately thereafter," the DAG report says.

The IPS also rejects claims by one independent expert, Professor John Daugman, of Cambridge university, that false fingerprint matches could reach a rate of one in 1,000.

It said Prof Daugman's research was based on ID card schemes collecting prints from two fingers, where the UK scheme would collect all ten.

The BAG report also raises concern about the "proliferation of different fingerprint readers" to be used by officials which it said could "lead to confusion".

And it urges the government to do more to protect privacy and make sure the public know their data protection rights - particularly as the Identity Cards Act allows some data to be shared with government departments without the consent of the individual.