LONDON — Several London hospitals said Tuesday that they had to cancel operations and send patients away because of a cyberattack on a company that supplies pathology laboratory services.

The firm, Synnovis, said it had been hit with a ransomware attack. Chief Executive Mark Dollar said the attack “has affected all Synnovis IT systems, resulting in interruptions to many of our pathology services.”

“It is still early and we are trying to understand exactly what has happened,” he said.

The National Health Service said there had been a “significant impact” at King’s College and Guy’s and St Thomas’ hospital trusts, which between them run several south London hospitals, as well as to clinics and doctors’ practices across a swath of the city.

A memo to staff said the “critical incident” had had a “major impact” on the delivery of services, with blood transfusions particularly affected.

NHS England London region said in a statement that it was “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.”

Oliver Dowson, 70, was prepared for an operation from 6 a.m. on Monday at the Royal Brompton Hospital, but was told by a surgeon at about 12.30 p.m. that it would not happen.

“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."

Ransomware attacks involve criminals paralyzing computer systems with malware, then demanding money to release them. Ransomware is the costliest and most disruptive form of cybercrime, crippling local governments, court systems, hospitals and schools as well as businesses. It is difficult to combat as most gangs are based in former Soviet states and out of reach of Western justice.

Britain’s state-funded health system has been hit before, including during a 2017 ransomware attack that froze computers at hospitals across the country, closing down wards, shutting emergency rooms and bringing treatment to a halt.

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