Major London hospitals cancelled some operations on Tuesday after a cyber attack on a key service supplier had a “major impact”, particularly on blood transfusions, according to internal memos posted on social media.

The attack on pathology partner Synnovis hit King’s College Hospital and Guy’s and St Thomas’ in the centre of the British capital.

“I can confirm that our pathology partner Synnovis experienced a major IT incident earlier today, which is ongoing,” Ian Abbs, CEO of Guy’s and St Thomas NHS Foundation Trust, wrote in the memo.

“This is having a major impact on the delivery of our services, with blood transfusions being particularly affected. “Some activity has already been cancelled or redirected to other providers,” he added.

The Health Service Journal (HSJ) trade publication reported that the system was the victim of a ransomware attack, citing senior sources. One of the sources said it could take “weeks, not days” to access pathology results.

The health service said it was working “urgently” with the country’s main cyber security agency, the National Cyber Security Centre (NCSC), and its own cyber operations team to fully understand the impact of the incident.

A cyber incident in May 2017, which disrupted businesses and government services worldwide, impacted more than a third of England’s 236 NHS trusts and led to the cancellation of an estimated 19,000 appointments in the span of a week.