The records were on six laptops stolen from St George’s Healthcare NHS Trust in Tooting around two weeks ago.
They had been locked in a cupboard over the weekend beginning Friday 6 June.
A statement from the trust said: ‘The laptops were stored in a secure room in a locked cabinet which appears to have been broken into by force.’
According to the trust, the files were password-protected and were likely to have been wiped by the thieves so the computers could be sold.
‘The Trust has written to every patient, apologising for the potential risk to their confidentiality,’ a spokesperson added.