Spanish telecommunications company Telefonica was among the many targets in Spain, Portugal Telecom and Telefonica Argentina were also targeted. It spreads from machine to machine silently and remains invisible to users until it unveils itself as so-called ransomware, telling users that all their files have been encrypted with a key known only to the attacker and that they will be locked out until they pay $300 to an anonymous party using the cryptocurrency Bitcoin.
Mrs May said: "The National Cyber Security Centre is working closely with NHS to ensure that they support the organisations concerned and that they protect patient safety". Sixteen U.K. organizations have been affected by the ongoing attack, according to the latest update from the NHS, which added that there is no evidence patient data has been accessed.
"It's an worldwide attack and a number of countries and organizations have been affected", she said.
Hackers exploiting data stolen from the US government conducted extensive cyberattacks on Friday that hit dozens of countries around the world, severely disrupting Britain's public health system and wreaking havoc on tens of thousands of computers elsewhere, including Russia's ministry for internal security.
According to Ryan Kalember, senior Vice President of cyber security strategy at the cybersecurity firm Proofpoint, a "ransomware worm" using the essentially unaltered NSA code is spreading across corporate networks in at least 74 countries, with European and Asian countries among the hardest hit.
Hospitals in London, northwest England and other parts of the country reported problems with their computer systems Friday.
Hospitals in the United Kingdom and telecommunications companies in Spain are among those hit by a "ransomware" attack that locked up computer data and demanded payment to free it.
He said the affected computers likely had not applied the Microsoft patch or were running old operating systems for which no patch was available.
"We are experiencing a major IT disruption and there are delays at all of our hospitals", said the Barts Health group, which manages major London hospitals.
"Seeing a large telco like Telefonica get hit is going to get everybody anxious".
Still, only a small number of US-headquartered organizations were hit because the hackers appear to have begun the campaign by targeting organizations in Europe, said Vikram Thakur, research manager with security software maker Symantec.
Russia's interior ministry said that some of its computers had been hit by a "virus attack" and that efforts were underway to destroy it.
PWC Cybersecurity partner Colin Slater said: "There's no evidence of data being taken from computers --this is purely a ransomware attack".
Friday's [14 April 2017] release - which came as much of the computing world was planning a long weekend to observe the Easter holiday - contains close to 300 megabytes of materials the leakers said were stolen from the NSA. Routine appointments had been cancelled and ambulances were being diverted to neighbouring hospitals.
Perhaps the most significant target thus far has been Britain's National Health Service, that nation's main healthcare apparatus.
Microsoft issued a security update on March 14 about vulnerabilities in the Windows system. It was among a large number of hacking tools and other files that a group known as the Shadow Brokers released on the Internet. In 2016, Hollywood Presbyterian Medical Center in California said it had paid a $17,000 ransom to regain control of its computers from hackers.
Agency spokeswoman Irina Volk says in a statement carried by Russian news agencies that Friday's cyberattacks hit about 1,000 computers.
"Undoing the hack is going to be just about impossible", he said. Carlos Cabreiro, the director of a police unit that fights cyber crime, told the newspaper Público that the country was facing "computer attacks on a large scale against different Portuguese companies, especially communication operators".
A trust spokesman said: "NUH is not affected by the national cyber security attack but has taken immediate measures to secure ICT systems".