The National Health Service (NHS) said on Friday 16 organisations had been affected by the "ransomware" attack.
It said in a statement that the NHS had not been specifically targeted, adding that the attack was affecting organisations from across a range of sectors.
"The investigation is at an early stage but we believe the malware variant is Wanna Decryptor," NHS said.
"At this stage we do not have any evidence that patient data has been accessed." Britain's National Cyber Security Centre, part of the GCHQ spy agency, said it was aware of a cyber incident and was working with NHS and the police to investigate.
'Major disuption'Hospitals across England reported the cyber attack was causing huge problems to their services and the public in areas affected were being advised to only seek medical care for emergencies.
A reporter from the Health Service Journal said the attack had affected X-ray imaging systems, pathology test results, phone systems and patient administration systems.
The Barts Health group, which manages major central London hospitals, including The Royal London and St Bartholomew's, said it had activated a major incident plan and had cancelled routine appointments.
"We are experiencing a major IT disruption and there are delays at all of our hospitals," it said.
"Ambulances are being diverted to neighbouring hospitals." Derbyshire Community Health Services said in a Twitter post: "We are aware of a major IT secure system attack. All IT systems have been temporarily shut down." Blackpool Hospitals NHS Trust, which includes six hospitals, said: "Please don't attend A&E unless it's an emergency", adding "Please avoid contacting your GP practice unless absolutely necessary".
There was no immediate comment from the health ministry or from Prime Minister Theresa May who was out campaigning in northeast England ahead of a general election on June 8.
Britain's opposition Labour Party said the attack on English hospitals showed the need to place cyber security at the heart of government policy.
"This incident highlights the risk to data security within the modern health service and reinforces the need for cyber security to be at the heart of government planning," Labour's health spokesman Jonathan Ashworth said.
Spain's government said on Friday a large number of companies, including telecommunications giant Telefonica , had been attacked by cyber criminals who infected computers with ransomware.