"A number of NHS organisations have reported to NHS Digital that they have been affected by a ransomware attack," the public health service said in a statement.
Britain's National Cyber Security Centre is assisting the investigation into the incident, apparently caused by a piece of malware called Wanna Decryptor, it said.
"At this stage we do not have any evidence that patient data has been accessed," the statement said, adding: "This attack was not specifically targeted at the NHS and is affecting organisations from across a range of sectors."
Several individual British health service trusts, each responsible for several hospitals, reported problems with their computer systems.
"We are aware of a major IT secure system attack. All IT systems have been temporarily shut down," Derbyshire Community Health Services said on Twitter.
Blackpool Hospitals NHS Trust, which includes six hospitals, said: "Please don't attend A&E unless it's an emergency" and "Please avoid contacting your GP practice unless absolutely necessary".
A spokesman for Barts Health NHS Trust in London said it was experiencing "major IT disruption" and delays at all four of its hospitals.
"We have activated our major incident plan to make sure we can maintain the safety and welfare of patients," the spokesman said.
"We are very sorry that we have to cancel routine appointments, and would ask members of the public to use other NHS services wherever possible.
"Ambulances are being diverted to neighbouring hospitals."
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)