Memos to NHS staff at King’s College and Guy’s and St Thomas’, say the delivery of blood transfusions has been badly affected.

Some operations have been cancelled.

Reports say accessing pathology results could take “weeks, not days”.

A spokesperson for NHS England London region said: “On Monday June 3 Synnovis, a provider of lab services, was the victim of a ransomware cyber attack.

“This is having a significant impact on the delivery of services at Guy’s and St Thomas’, King’s College Hospital NHS Foundation Trusts and primary care services in south-east London and we apologise for the inconvenience this is causing to patients and their families.

“Emergency care continues to be available, so patients should access services in the normal way by dialling 999 in an emergency and otherwise using 111, and patients should continue to attend appointments unless they are told otherwise. We will continue to provide updates for local patients and the public about the impact on services and how they can continue to get the care they need.

“We are working urgently to fully understand the impact of the incident with the support of the government’s National Cyber Security Centre and our cyber operations team.”