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The cards are used to access patient records in a new computerised record system built by Connecting for Health (CfH), the NHS IT group. Both a smart card and a six digit PIN are required to access the system, so a lost card alone will not necessarily provide access.

"There is no evidence that any security breaches have ever arisen from lost of stolen cards," a CfH spokesman told the BBC.

Pulse magazine, a periodical aimed at GPs, requested the information under the Freedom of Information Act, and found that CfH had issued 500,000 such cards, and lost over 4,000.

Most were simply lost by staff, although 142 of the cards were found to have been stolen. One in ten NHS trusts reported that they had no idea how many cards had been lost.

Security experts have expressed doubt over the security of a system that needs to allow hundreds of thousands of workers access, and a recent British Medical Association poll found that nine out of 10 doctors had no confidence in the safety of patient's data online.