Sensitive data from a ransomware attack that led to major disruption at London hospitals has been published online, UK health authorities said on Friday.

The cyberattack earlier this month on service supplier Synnovis particularly hit blood transfusions, and hundreds of appointments and operations are still being cancelled.

The attack affected some of the UK's biggest hospitals -- King's College Hospital and Guy's and St Thomas' in the centre of the British capital.

"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," said the body that runs the state-run National Health Service (NHS) in England.

According to the BBC, Russian cyber criminal group Qilin shared almost 400 gigabytes of data -– including patient names, dates of birth, NHS numbers and descriptions of blood tests –- on their darknet site and Telegram channel.

Synnovis, which conducts pathology tests on blood samples, primarily in southeast London, was the victim of a cyberattack on June 3.

"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," NHS England said.