Update - 7.45pm: At least 16 hospitals in the UK's National Health Service have been targeted by a large cyber attack.
The New York Times reports that "Attacks were being reported in Britain and 11 other countries, including Turkey, Vietnam, the Philippines, Japan, with the majority of affected computers in Russia."
IT systems of around 40 organisations across the NHS were affected by a ransomware attack, in which malicious software encrypts the data on a victim's computer and demands a ransom, usually to be paid in Bitcoin, in order to make the data accessible again.
It appears the bug was delivered via email which had an encrypted, compressed file attached. Once loaded, the hackers had access to the system.
The bug was among the leaked NSA hacking tools. Microsoft has rolled out a patch but the affected hospitals hadn't updated their systems.
Earlier: The NHS in England is investigating "an issue with IT" amid reports of a cyber attack on its systems.
Hospital trusts and GP groups in Lancashire and Hertfordshire were among those reporting problems, with one warning patients to only visit hospital accident and emergency departments "if absolutely necessary".
Blackpool Teaching Hospitals NHS Trust and the resort town's clinical commissioning group (CCG) warned of problems.
The CCG tweeted: "We are aware of an IT issue affecting some GP computer systems.
"Patients are asked for understanding whilst the issue is resolved.
‼️ Important notice pic.twitter.com/B52Agc0oRd— Fylde Coast CCGs (@FyldeCoastCCGs) May 12, 2017
"Please avoid contacting your GP practice unless absolutely necessary. Should you wish to obtain non-urgent medical advice, please call 111.
"Please also only attend the Walk-In Centre and A&E department if absolutely necessary."
A spokesman for NHS England said there was "an issue with IT", but referred further inquiries to NHS Digital, which did not immediately comment.
It comes months after Barts Health Trust, the largest NHS trust in England, was hit by a ransomware cyber attack.
The trust sent a message to staff urging them not to open email attachments from unknown senders.
Viruses such as ransomware are normally delivered via emails which trick the recipient into opening attachments and releasing malware onto their system in a technique known as phishing.
Derbyshire Community Health Services Trust tweeted: "We are aware of a major IT secure system attack.
"All IT systems have been temporarily shut down. More information will be available shortly."
A Barts spokesman said it was experiencing "major IT disruption" and delays at all four of its hospitals, The Royal London, St Bartholomew's, Whipps Cross and Newham.
He said: "We have activated our major incident plan to make sure we can maintain the safety and welfare of patients.
"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."
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.
The incident has spread to Scotland after three GP surgeries in Dumfries and Galloway were hit.
A Dumfries and Galloway healthboard spokesman said: "Three GP practices have been initially affected and we are taking precautionary measures to prevent any others being affected."
He declined the name the practices involved and said the board was "comfortable and confident" with the steps taken but added "we don't know what we're dealing with".
He said: "We are monitoring the situation here as are all health boards in Scotland."