GP systems across whole areas are understood to have been disrupted by the attack. Systems affected by the malware attack have had messages appear on their screens demanding payment to unlock files.
The message demands $300 in the virtual currency Bitcoin and warns that failure to pay within three days will double the fee, and that files will be unrecoverable after seven days.
A GP surgery in Bury warned on Twitter that practices across Greater Manchester had seen their IT systems disabled by the attack.
All Greater Manchester networks down -we cannot access any patient info plz RT @NHSBuryCCG— Peel GPs (@peelgps) May 12, 2017
One GP practice contacted GPonline via Twitter to say that 'all computers' were shut down at the practice, and that staff were implementing a business continuity plan.
An NHS Digital spokeswoman said: 'A number of NHS organisations have reported to NHS Digital that they have been affected by a ransomware attack which is affecting a number of different organisations.
'The investigation is at an early stage but we believe the malware variant is Wanna Decryptor.
'At this stage we do not have any evidence that patient data has been accessed. We will continue to work with affected organisations to confirm this.
'NHS Digital is working closely with the National Cyber Security Centre, the Department of Health and NHS England to support affected organisations and to recommend appropriate mitigations.
'This attack was not specifically targeted at the NHS and is affecting organisations from across a range of sectors.
'Our focus is on supporting organisations to manage the incident swiftly and decisively, but we will continue to communicate with NHS colleagues and will share more information as it becomes available.'
Patients said on social media that they had been turned away from GP appointments because their practices had been affected by the cyber attack. GPs highlighted that without access to patient records and notes on practice IT systems, their ability to deliver normal services to patients was severely affected.