The data was published by criminal gang Qilin following its attack on pathology services run by Synnovis at Guy's and St Thomas' NHS Foundation Trust and King's College Hospitals NHS Trust.

A Synnovis spokesperson said on 21 June: ‘On 3 June, Synnovis – a pathology services partnership between two London-based hospital trusts and SYNLAB – was the victim of a cyber attack. Last night a group claiming responsibility for the cyberattack published data online that they allege belongs to Synnovis.

‘We know how worrying this development may be for many people. An analysis of this data is already underway. This analysis, run in conjunction with the NHS, the National Cyber Security Centre and other partners, aims to confirm whether the data was taken from Synnovis' systems and what information it contains.

‘We will keep our service users, employees and partners updated as the investigation progresses.'

The BBC reported that the published data included patient names, dates of birth, NHS numbers and descriptions of blood tests as well as business account spreadsheets detailing financial arrangements between hospitals and GP services and Synnovis.

The attack has caused huge disruption with 1,134 elective procedures and 2,194 outpatient appointments at the two hospitals.

Dr Chris Streather, medical director for NHS London, said: ‘Although we are seeing some services operating at near normal levels and have seen a reduction in the number of elective procedures being postponed, the cyber-attack on Synnovis is continuing to have a significant impact on NHS services in south east London.'