Critically ill patients are being diverted to unaffected hospitals as computer systems failed in Accidents & Emergency (A&E) units and doctors were locked out of test results, Xrays and patient records.
The spread of the ransomware appeared to have stopped on Saturday after a security researcher registered a domain name connected to the malware.
By the time they turned their attention to the United States, spam filters had identified the new threat and flagged the ransomware-laden emails as malicious, Thakur said.
It is feared computers in A&E wards, GP surgeries and other vital services across the NHS were infected with a virus based on hacking tools developed by U.S. cyber warfare agents.
Markus Jakobsson, chief scientist with security firm Agari, said that the attack was "scattershot" rather than targeted.
"The age-old advice is to never click on a link in an email", said Jerome Segura, a senior malware intelligence researcher at Malwarebytes, a San Jose-based company that has released anti-ransomware software.
Answer: It's too early to say who was behind this, ransomware is usually associated with highly organised criminal gangs.
The NSA had developed the "Eternal Blue" weapon to gain access to computers around the world but the tool was stolen and leaked by Shadow Brokers.
Microsoft released a patch for the issue in March but computers that have not been updated are still vulnerable to the attack.
Before Friday's attack, Microsoft had made fixes for older systems, such as 2001's Windows XP, available only to mostly larger organizations that paid extra for extended technical support.
Spain, Ukraine and India were also severely affected, according to researchers from the Kaspersky Lab.
Hospitals in areas across Britain found themselves without access to their computers or phone systems.
A fifth of the trusts were hit by the ransomware on Friday afternoon, forcing hospitals to cancel and delay treatment.
English hospitals and clinical commissioning groups (CCGs) in London, Blackpool, Hertfordshire and Derbyshire were among those to report problems.
The spokesman added that there was no production scheduled to begin today.
NHS Digital said the attack "was not specifically targeted at the NHS and is affecting organizations from across a range of sectors".
Home Secretary Amber Rudd is to chair an emergency meeting as tech experts work to disrupt an worldwide cyber attack which has crippled NHS services across the country.
Dr Krishna Chinthapalli, a neurology registrar at the National Hospital for Neurology and Neurosurgery in London, had warned that an increasing number of hospitals could be shut down by ransomeware attacks in an article on the vulnerability of the NHS network in the "British Medical Journal" on Wednesday, two days before the major cyber-hack.
"All necessary steps are being taken to ensure that the cause and nature of this attack is identified".
"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.
In Spain, major companies including telecommunications firm Telefónica were infected.
How do ransomware attacks take place? The speed and scale of the malware attack has reportedly startled experts. That was a shock.