Barts health trust, which serves almost 2.5million people across east London warned staff in its five hospitals that it was undergoing an attack.
“We are urgently investigating this matter and have taken a number of drives offline as a precautionary measure, a Barts spokeswoman said. “We have tried and tested contingency plans in place and are making every effort to ensure that patient care will not be affected.”
It was reported earlier on Friday that the trust had been targeted with ransomware, which is normally delivered via emails that trick the recipient into opening attachments and releasing malware on to their system. But the spokeswoman ruled out such an attack on Friday night.
The trust has not said how much of its system has been affected by the attack or whether patient data has been compromised but it said it believed that most of the affected system was housing corporate data.
The trust’s filing system between departments has been turned off while the investigation takes place.
Staff at the Royal Free London foundation trust were also warned to beware of attacks on Friday, the Guardian has learned.
“We have been informed of a major cyber-attack on NHS organisations. Please exercise extreme caution when opening any email attachments from unknown source or that don’t seem relevant to you. We will be carrying out security scans on all computers within the trust so please leave them switched on until further notice,” wrote the trust’s IT director, Tosh Mondal.
NHS Digital said it was aware that Barts had been infected by a “virus which has affected their IT systems”.
In October, the Northern Lincolnshire and Goole foundation trust was hit by an attack in which malware was used to encrypt files and demand a ransom in order to restore access. The trust did not pay the ransom but was forced to cancel patient appointments as its systems were shut down to remove the virus.