Three London hospitals lost email and internet access yesterday after a virus attack.
St Bartholomews, the Royal London hospital in Whitechapel and the London Chest hospital in Bethnal Green, were declared in a 'state of emergency' after the virus, which is thought to be the Mytob bug, hit.
The virus acts as a 'back door' that allows hackers to gain access to the hospital computers and even patient databases.
Back-up systems were used to ensure operating theatres and outpatient clinics were kept open, while ambulances were diverted to other London hospitals.
"There is no indication that this was a malicious attack, but engineers are still investigating where the virus came from and what it is," said an NHS spokesman.