Six laptops have been stolen from a major teaching hospital, Scotland Yard said today.
The laptops contain information about some 20,000 patients, including their names, date of birth and postcodes, a spokeswoman for St George's Hospital, in Tooting, south London, said.
She said: "The Trust has written today to every patient, apologising for the potential risk to their confidentiality.
"All data on the laptops was password-protected and personal information such as postcode was hidden, although the patient's name and hospital number was shown.
"The Trust acknowledges that patient data should not have been stored on laptops. This was done as a temporary measure because of a problem with the computer network.
"However, the laptops were in a secure area under lock and key. The data was being used to monitor and reduce waiting times at the hospital."
The six laptops were stolen from offices in the Atkinson Morley Wing of the hospital between the evening of Friday 6 June and the morning of Monday 9 June.
The spokeswoman said: "The laptops were stored in a secure room in a locked cabinet which appears to have been broken into by force."
The theft was discovered on Monday morning and the police were contacted.
The spokeswoman said the hospital trust has begun its own investigation into the circumstances of the theft and any recommendations regarding security will be implemented immediately.
Chief executive David Astley said: "We offer all our patients our sincere apologies for putting their confidential information at risk, although we could not anticipate a determined thief who was prepared to force open a filing cabinet and locked drawers to get to the laptops.
"We believe the data will almost certainly be wiped by the thief so he can get a quick sale.
"Nonetheless we owe it to our patients to protect their personal information and we have reminded our staff not to store this kind of data on laptops in the future.
"We have also set up a helpline for patients to ring for further information."
The helpline number has been given to patients who are affected in the letter which was sent today. Patients who do not receive a letter are being told not to worry as their details are not on the stolen laptops.
Mr Astley added: "We have not lost any patient data as a result of this incident. The Trust still holds their records on a secure central system.
"This theft will not disrupt any treatments or appointments planned for the patients affected."
A spokesman for the Metropolitan Police said: "We have received an allegation of the theft of six laptop computers from St George's Hospital in Tooting.
"It was reported to us on Monday, 9 June 2008.
"It is being investigated by the Wandsworth Burglary Unit. Anyone with information can contact them on 0208 247 8745."