Thousands of patient files could be affected after England's largest NHS trust was hit by a cyber-attack.
Barts health trust, which runs five hospitals in east London, is urgently investigating the security breach which was discovered on January 13.
Ransomware has been ruled out as the root cause but it is currently unclear how much data has been compromised.
Staff have been told not to open email attachments from unknown senders and several drives have been taken offline as a precautionary measure.
A spokesperson for Barts said: "We have tried and tested contingency plans in place and are making every effort to ensure that patient care will not be affected."
The trust - which was also hit by a cyber-attack in 2008 - runs Royal London in Whitechapel, Whipps Cross in Leytonstone, Newham in Plaistow, Mile End and St Bartholomew's in the City.
Speaking to the Guardian, John Bambenek, a threat intelligence manager at the firm Fidelis Cybersecurity, said: “The trouble is that local authorities and governments aren’t very prepared and they have extremely valuable information that simply can’t be lost, so they’re a tempting target for cybercriminals."