England's largest NHS Trust has been hit by a cyber attack, it has emerged.
Barts Health Trust, which runs The Royal London, St Bartholomew's, Whipps Cross, Mile End and Newham hospitals, is investigating the breach.
The trust said it could now rule out ransomware, in which email recipients are tricked into opening attachments which contain viruses, as the cause.
It has not confirmed how much of its system was affected but said there was no sign that patient data was accessed.
In a statement said: "We are urgently investigating this matter and have taken a number of drives offline as a precautionary measure.
"We have tried and tested contingency plans in place and are making every effort to ensure that patient care will not be affected."
The incident follows a similar attack on the Northern Lincolnshire and Goole Foundation Trust in October, when malware was used to encrypt files on the trust's system and demand a ransom in order to access them again.
The trust did not pay out, but was forced to cancel patient appointments while its systems were shut down to remove the virus.