The largest NHS hospital trust in England has been infected with a virus affecting thousands of sensitive files.

A message sent to staff at Barts Health Trust on Friday morning warns that the trust’s four hospitals are experiencing a “ransomware virus attack.”

The victims of blackmail ransomware usually receive phishing messages in an attempt to elicit money in order to restore access to the computer system.

The trust said it was making “every effort” to avoid disruption to patients treated at its four hospitals in east London: The Royal London, St Bartholomew’s, Whipps Cross and Newham.

A source at the trust told Health Service Journal that the attack had affected thousands of files on the trust’s Windows XP operating system, and the trust’s file sharing system between departments has been turned off while an investigation takes place.