Major hospitals in London have declared a critical incident after a cyber attack led to the cancellation of operations and the loss of blood transfusions.

Memos to National Health Service staff at King's College Hospital, Guy's and St Thomas' and primary care services in the UK capital say pathology partner Synnovis was hit by a "major IT incident".

Synnovis is a provider of pathology services and was formed from a partnership between SynLab UK & Ireland, Guy's and St Thomas' NHS Foundation Trust and King's College Hospital NHS Foundation Trust.

The memos said the "critical incident" has had a "major impact" on the delivery of services, with blood transfusions particularly affected.

Some procedures and operations have been cancelled or have been redirected to other NHS providers as hospital bosses continue to establish what work can be carried out safely.

One patient, Oliver Dowson, 70, was prepared for an operation from 6am on Monday at Royal Brompton when he was told by a surgeon at about 12.30pm 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 told the PA news agency.

"I've been given a date for next Tuesday and am crossing my fingers - it's not the first time that they have cancelled, they did it on May 28 too, but that was probably staff shortages in half-term week."

A spokesman for King's College Hospital in London confirmed it was affected by the attack.

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

In 2021, it was announced that SynLab would partner with the NHS to deliver pathology services at hospitals and GP services across southeast London.