Prime Minister Theresa May said the ransomware hit was "not targeted" at the health service but was part of a wider assault on organisations across a number of countries.
Employees at Telefonica reportedly saw warnings on their screen largely identical to the ones that appeared on screens at British hospitals, demanding that a ransom be paid in bitcoin in return for access to their files. There were no immediate reports of victims in the US.
Hospitals and GP surgeries in England and Scotland were among 16 health service organisations hit by a ransomware attack, using malware called Wanna Decryptor.
Hospitals affected are believed to have lost the use of phone lines and computers, with some diverting all but emergency patients elsewhere.
The NHS has confirmed that a ransomware attack is responsible for major IT problems affecting a number of hospitals. Some chemotherapy patients were even sent home because their records could not be accessed.
UPDATE-Attackers have hit several hospitals in the United Kingdom, along with major corporations in Spain and other countries with a ransomware attack, disrupting network and phone operations and forcing some of the hospitals to postpone non-emergency services and divert patients to other facilities.
The National Health Service (NHS) in the United Kingdom has confirmed that 16 different NHS organizations have been affected by the brand-new Wanna Decryptor 2.0 ransomware-while it has emerged that the campaign is global and coordinated, with tens of thousands of attacks on various sectors in just the last few hours.
"Both staff and patients were frankly pretty appalled that somebody, whoever they are, for commercial gain or otherwise, would attack a health care organization", he said.
"We commend NHS England on reacting so quickly, and for being open about this attack". Similar attacks have been taken place around the world.
"Emergency operations may have been cancelled, and automated systems that help monitor in and outpatients could be affected as well", Wool said. In addition, the No More Ransom Project also includes an online tool that enables ransomware victims to learn if a free decryptor is available by uploading a single encrypted file.
Spain's Telefonica was among the companies hit.
South Tees, which runs James Cook and the Friarage Hospitals, said it is not affected, stating: "We have not been infected by the virus and we are doing everything we can to ensure our systems remain as secure as possible".
An NHS Western Isles spokeswoman said: "We can confirm that we have been affected but can't confirm to what extent at the moment".
He said most likely it occurred this time because some of the hospitals and other organizations affected may not have applied a patch that Microsoft released or they are using outdated operating systems no longer supported by the software giant.
British government officials and intelligence chiefs have repeatedly highlighted the threat to critical infrastructure and the economy from cyberattacks.
A year ago a Sky News investigation discovered that NHS trusts were putting patients at risk, with seven trusts spending nothing on cybersecurity in 2015.