NHS Digital says the attack "was not specifically targeted at the NHS and is affecting organizations from across a range of sectors".
Wanna Decryptor is a piece of malicious software that encrypts files on a user's computer, blocking them from view and threatening to delete them unless a payment is made.
Dr Tony Naughton, the chief clinical officer at Fylde and Wyre CCG, described the problem as "national", while East and North Hertfordshire NHS Trust said it believed it had been hit by a "cyber attack" and had suspended all non-urgent activity.
But those attacks - blamed on Russian Federation, which has repeatedly denied them - followed an entirely different modus operandi involving penetrating the accounts of individuals and political organizations and then releasing hacked material online.
Britain's opposition Labour Party said the attack on English hospitals showed the need to place cyber security at the heart of government policy.
Power firm Iberdrola and utility provider Gas Natural were also reported to have suffered from the outbreak.
"At this stage we do not have any evidence that patient data has been accessed", the system says.
A global ransomware attack is holding more than 60,000 computers hostage.
Crystal Palace vs Hull City - fans' view
It was a mixed bag of a season for manager Bert Head's Palace , who finished seventh and comfortably above 12th-placed Hull City. I like to believe that will happen on Sunday". "We will prepare for these games like we prepare for the others", Silva added.
Whipps Cross Hospital, part of Barts Health Trust, has had to cancel routine appointments as a result, with hospital staff activating a "major incident plan".
The hackers are reportedly exploiting a critical vulnerability (MS17-010) that first received a patch on 14 March.
Screenshots of WannaCry with text in Spanish were also shared online.
In response to the attack Jonathan Martin, EMEA Operations Director at Anomali said: "This was an vulnerability waiting to happen".
"This was not targeted at the NHS, it's an global attack and a number of countries and organizations have been affected", Prime Minister Theresa May said.
"NHS Digital is working closely with the National Cyber Security Centre, the Department of Health and NHS England to support affected organizations and to recommend appropriate mitigations". Attacks occur regularly against Telefonica and today's wasn't major, the official said.
Affected NHS trusts said that IT systems had been shut down in order to protect them. Attacks of this kind have spiked in the previous year, jumping from 340,665 attacks in 2015 to 463,841 in 2016, according to Symantec.
Some frightening news out of the United Kingdom: someone has hacked the computers of the National Health Service and is demanding ransom.
- Stephen Colbert responds to President Trump's 'no-talent' insult
- President Trump Demands 'Goddamned Steam' Catapults in Time Interview
- April : 8 MI teams earn flawless multi-year scores
- Russia's Top Diplomat Meets With Trump at White House
- No need for Mumbai Indians to go into panic mode: Kieron Pollard
- Bottas was always going to be quick, says Hamilton
- Karnan seeks SC reprieve, stays clear of police radar
- Britain WILL send soldiers back to Afghanistan - and THIS is how many
- New Samsung Galaxy Folder Smartphone Being Developed
- Consistent Chelsea Deserve Top Position, Says West Brom's Pulis