Global cyber attack hits at least 99 countries

"This is one of the largest global ransomware attacks the cyber community has ever seen", said Rich Barger, director of threat research with Splunk, one of the firms that linked WannaCry to the NSA.

Hospitals across Britain found themselves without access to their computers or phone systems.

Most cases have involved desktop computers in GP surgeries, dental practices and other primary care sites, but hospitals in the NHS Lanarkshire region are also affected.

Spain's Telefonica was among the companies hit.

British Prime Minister Theresa May says a cyberattack initially believed to be targeting only hospitals in the United Kingdom has now gone beyond, involving potentially dozens of countries.

According to a report by the Guardian, security researchers with Kaspersky Lab have recorded more than 45,000 attacks in 99 countries, including the UK, Russia, Ukraine, India, China, Italy, and Egypt.

What's caused the attack?

It encrypts files on a user's computer, blocking them from view, before demanding money, via an on-screen message, to access them again.

The virus is usually covertly installed on to computers by being hidden within innocent-looking emails containing links, which users are tricked into opening.

Gangs are thought to have used tools stolen from USA spies and dumped online by hackers linked to Russian Federation.

A hacking group called Shadow Brokers released the malware in April claiming to have discovered the flaw from the NSA, Kaspersky said.

Mr Martin urged the public to reduce the risk of an attack by making sure the security software patches are up to date, by running proper anti-virus software and, most important of all, by backing up data.

Computers in thousands of locations have been locked by a programme that demands 300 Dollars in Bitcoin.

Among the worst hit was the National Health Service (NHS) in England and Scotland.

"But our commitment is we will be as open as we can be, as soon as we can be, as our investigation continues".

The National Cyber Security Centre quickly stepped in to co-ordinate nationally and advised: "The picture is emerging that this is affecting multiple countries and sectors and is not exclusively targeted at the NHS".

It is understood that several health trusts turned their computer systems off as a precautionary measure, rather than being shut down by the attack.

What did Theresa May say in response to the attack?

It remains unclear exactly how this ransomware strain is being disseminated and why it appears to have spread so quickly, but there are indications the malware may be spreading to vulnerable systems through a security hole in Windows that was recently patched by Microsoft.

Home Secretary Amber Rudd, who is to chair an emergency COBRA meeting to discuss the crisis later, could not confirm if all patient files had been backed up.

The attacks began around lunchtime but it was not until late afternoon that the CareCert NHS cyber security guidance service run by NHS Digital issued an advisory identifying the ransomware variant and offering advice on the Microsoft patch that NHS organisations needed to be applied.

The National Cyber Security Centre is working closely with NHS Digital to ensure they support the organisations concerned and that they protect patient safety.

"And, we are not aware of any evidence that patient data has been compromised".

How has it affected hospital patients?

As the scale of the security breach became clear on Friday afternoon, ambulances were diverted and patients told to avoid some A&E departments.