Major hospitals in London have declared a critical incident after a cyber attack led to operations being cancelled and emergency patients being diverted elsewhere.

It applies to hospitals partnered with Synnovis – a provider of pathology services.

King’s College Hospital, Guy’s and St Thomas’ – including the Royal Brompton and the Evelina London Children’s Hospital – and primary care services are among those affected.

The incident has had a “major impact” on the delivery of services, especially blood transfusions and test results.

Some procedures have been cancelled or have been redirected to other NHS providers as the hospitals try to establish what work can be carried out safely.

A spokesman for King’s College Hospital confirmed it had been affected by the attack.

The incident is thought to have occurred on Monday, meaning some departments could not connect to a main server.

According to the Health Service Journal (HSJ), several senior sources have told it the system has been the victim of a ransomware attack.

One said gaining access to pathology results could take “weeks, not days”.

There are suggestions urgent and emergency care at the hospitals will be affected as they may not be able to access quick-turnaround blood test results.

One patient, Oliver Dowson, 70, was prepared for an operation from 06:00 at Royal Brompton. He was told by a surgeon at about 12:30 that it would not be going ahead.

“The staff on the ward didn’t seem to know what had happened, just that many patients were being told to go home and wait for a new date,” he said.

“I’ve been given a date for next Tuesday and am crossing my fingers.

“It’s not the first time that they have cancelled, but that was probably staff shortages in half-term week.”