File photo: The NHS London Ambulance Service sign is seen on an ambulance in London. Image Credit: REUTERS

London: Britain's National Health Service said on Friday it was investigating claims that hackers had published confidential data stolen from several London hospitals in a cyber attack on a blood testing lab.

Services at large London hospitals including Guy's, St Thomas' and King's services were still being disrupted following the attack by hackers reportedly demanding ransom from Synnovis, a lab company which provides testing services, on June 3.

"NHS England has been made aware that the cyber criminal group published data last night which they are claiming belongs to Synnovis and was stolen as part of this attack," the state-run unit said.

"We understand that people may be concerned by this and we are continuing to work with Synnovis, the National Cyber Security Centre and other partners to determine the content of the published files as quickly as possible." At the affected hospitals, test processing is still at reduced levels and more than 1,000 elective procedures and 2,000 appointments have been cancelled since the incident.

The cyber criminal group behind the attack has been trying to extort money from lab company Synnovis and had threatened to publish data if it was not paid, the BBC reported on Friday.

It said sensitive data including names, dates of birth and NHS numbers had been published on the Telegram channel on Thursday night.