Tony Blair was forced onto the defensive today after a major report on plans for national identity cards concluded that they are "not feasible".

The London School of Economics study, published today, said that the overwhelming view of the experts who studied the government scheme was that it was too complex, technically unsafe and overly prescriptive.

At an estimated cost of £10billion-£19 billion it was also going to prove considerably more expensive than the £6-7 billion that the Government had claimed, said the report, The Identity Project: An Assessment Of The UK Identity Cards Bill And Its Implications.

Researchers who presented their findings today dismissively rejected the scheme as "a dog's dinner" and a "one-stop-shop for fraudsters" that was doomed to failure.

Faced with a torrent of negative analysis, Mr Blair issued a plea for the public not to write the cards off.

"All I'm saying to people is to keep an open mind on this enabling legislation," he said at his monthly Downing Street press conference.

"There's plenty of time for this debate to develop. All I'm saying is, understand why the Government is doing this at the moment - it's not to give ourselves a political headache.

"People recognise the benefits of a scheme that will allow us to tackle identity fraud more effectively, bear down on illegal working, illegal immigration, abuse of our public services and help in the fight against organized crime and terrorism."

MPs are due to vote tomorrow on the second reading of the Government's Bill to introduce identity cards based on biometric details such as fingerprints and iris scans. An almost identical Bill was thrown out by Parliament before the election.

The six-month LSE study, run by 14 professors who have consulted with 100 industry representatives, experts and researchers, gives warning of at least six potential flaws in the Government's scheme.

No ID-card scheme of such a size based on biometric data has ever been tried anywhere else in the world, it points out. The much smaller schemes that have been tried have suffered considerable technical problems, which would only be magnified in a larger scheme.

Researchers estimate that about 4-5 per cent of the population are what is technically described as "goats", or people who simply do not fit the technology. David Blunkett, the Work and Pensions Minister, is a prime example because his blindness means his irises cannot be scanned.

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