The NHS has been hit by a major cyber attack on its computer systems.
Ambulances have been diverted and patients warned to avoid some A&E departments after systems were targeted on Friday.
Some North East services are said to be among those affected, though no hospitals so far have reported issues.
NHS Digital said 16 organisations within the health service had seen their IT systems affected as of 3.30pm by a ransomware attack, using malware called "Wanna Decryptor".
An NHS Digital spokesman said: "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."
He added the attack "was not specifically targeted at the NHS and is affecting organisations from across a range of sectors".
Pictures posted on social media showed screens of NHS computers with images demanding payment of 300 US dollars worth of the online currency Bitcoin, saying: "Ooops, your files have been encrypted!"
It adds: "Maybe you are looking for a way to recover your files, but do not waste your time."
It demands payment in three days or the price is doubled, and if none is received in seven days the files will be deleted.
Hospitals and clinical commissioning groups (CCGs) in London, Blackpool, Hertfordshire and Derbyshire were among those to report problems.
St Barts Health NHS Trust, which runs The Royal London, St Bartholomew's, Whipps Cross and Newham hospitals in London, said it had implemented its major incident plan to cope with disruption.