Experts at Cambridge University believe the system is “broken” after they tricked it into accepting transactions without using a valid personal identification number.

They say the flaw is so fundamental it threatens to undermine the entire security of the system.

Which? said that in a recent survey 14 per cent of people – one in seven – said they have had money taken from their bank account and 13 per cent from a credit card.

Of those around half did not get their money back from the bank, even though they insisted they did not use or authorise the “disputed withdrawal”.

Professor Ross Anderson, a computer expert, claimed to have developed a way of bypassing security systems which renders chip and pin no longer “fit for purpose”.

His team’s “man in the middle” technique involves having a separate card reader in a back pack.

The fraudster puts the stolen credit or debit card into the shop’s reader but then the second reader in his bag sends a “pin OK” signal to the shop terminal.

The shop terminal then sends back a transaction go-ahead signal to the terminal with the stolen card and money is taken off it.

They claim to have used it a number of times without the banks, who firmly deny it is a threat, being any the wiser.

Prof Anderson, who two years proved that cards could be cloned, accused the banks of lying when they said there was no problem.

“How would the banks know if the fraud has taken place?,” he said. “We used it in our canteen and they have never contacted us to say that there was a fraud.

“We have many examples of people who have had their cards stolen and then purchases made using the chip and pin.

“They are adamant they didn’t use it but if the banks say chip and pin has been used you have to pay.

“I think many of these people would have been victim of the kind of technique we have developed.

“The banks are wrong. All the banks are lying. They are maliciously and wilfully deceiving the customer. If there was any justice then the police would be looking into this. The system is not fit for purpose.”

Stephen Mason, a barrister who specialises in cases of chip and pin fraud, said at the very least the potential breach of security should be investigated.

“There is clearly something wrong with the system,” he said. “I have people who believe as much as £4,000 has been taken from their accounts.

“It is not right for the banks to say our systems are perfect. The banks need to realise there is reasonable doubt about how secure the system is.”

But The UK Cards Association, which represents the banks’ card operations, dismissed the claim, saying that while the research had shown what it was possible to do in theory, this did not mean it was practical or even possible to do in reality.

Figures for fraud committed on lost and stolen cards were down to the lowest figure for two decades, she added.

The Financial Ombudsman Service, which decides on any disputed claims, said any new methods of committing fraud would be taken into consideration in future disputes.

At present, the customer is accountable for the fraud as banks argue that PIN verified transactions are secure. Given this attack demonstrates a clear method of bypassing the PIN system, this assertion by the banks stands on shakier ground.