NHS Merseyside, which operates a number of hospitals in northwestern England, tweeted, "we are taking all precautionary measures possible to protect our local NHS systems and services". People in affected areas were being advised to seek medical care only in emergencies.
A spokesman for Barts Health NHS Trust in London said it was experiencing "major IT disruption" and delays at all four of its hospitals.
The UK's state-run National Health Service declared a "major incident" after the attack, which forced some hospitals to divert ambulances and scrap operations.
The Spanish government said several companies including Telefonica had been targeted in ransomware cyberattack that affected the Windows operating system of employees' computers.
Following the cyber attacks pictures were posted on social media showing screens of NHS computers with images demanding payment of Dollars 300 (275 euros) worth of the online currency Bitcoin, saying: "Ooops, your files have been encrypted!"
NHS Digital, which oversees United Kingdom hospital cyber security, says the attack used the Wanna Decryptor variant of malware, which infects and locks computers while the attackers demand a ransom.
The malware's name is WCry, but analysts were also using variants such as WannaCry. A physician in Liverpool told the Guardian that his unit manually severed its connection to the broader NHS system in an attempt to stave off the infection.
It is the first piece of self-spreading ransomware, said Adam Meyers, a research with cyber security firm CrowdStrike. Expert Marco Cova has claimed it may derive from a tool known as EXTERNALBLUE which was developed by the US National Security Agency (NSA).
That exploit was one of many hacking tools stolen from the NSA and later published online by a group that called itself the Shadow Brokers, according to Avast.
He said the affected computers likely had not applied the Microsoft patch or were running old operating systems for which no patch was available. Organisations or individual users who failed to apply that patch to Windows machines may remain vulnerable to WannaCry.
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She said she should have paid more attention and been involved with the dealings of her professional and personal finances. Characterized as a defender for those in her district, she had a reputation for doing what she said she would.
Along with Britain's NHS, authorities in Spain also reported being hit by the ransomware attack. "We believe it to be ransomware". That gives you an idea about why the level is risk is particularly high. On Wednesday, hackers disputed the websites of several French media companies and aerospace giant Airbus (AIR.PA).Also, the hack happened four weeks before a British parliamentary election in which national security and the management of the state-run National Health Service (NHS) are important campaign themes.Authorities in Britain have been braced for possible cyberattacks in the run-up to the vote, as happened during last year's US election and on the eve of this month's presidential vote in France.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.
Mrs May said: "This is not targeted at the NHS, it's an global attack and a number of countries and organisations have been affected".
"At this stage we do not have any evidence that patient data has been accessed".
He said the goal of his BMJ article was to raise awareness of the NHS' vulnerability and encourage hospitals to take steps to prevent an attack.
Spain's Telefónica and Russia's MegaFon were among the targets.
The attack hit Britain's health service, forcing affected hospitals to close wards and emergency rooms.
Dr Chinthapalli, who had his article, "The hackers holding hospitals to ransom", published in the British Medical Journal (BMJ) on Wednesday, described NHS organisations as the "ideal victims" of cyber attacks, and said dozens of smaller hacks had happened in the past.
Spain's Telefonica was among the companies hit. "They said they started the system again". "And some of them may not be well prepared for such attacks", Camacho said.
The ransomware encrypts data on the computer using an encryption key that only the attacker knows.
Some chemotherapy patients were even sent home because their records could not be accessed.
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