Wainwright said the agency is analyzing the virus and has yet to identify who is responsible for the attack.
The attack, first detected on Friday, spread aggressively to hit computer networks run by governments, hospitals and businesses across the globe, with FedEx and Hitachi among those hit over the weekend.
Although experts had voiced fears that a second wave of attacks could strike systems on Monday, the British authorities said this appeared to have been avoided.
One of the variants, discovered by researcher Benkow, was reverse-engineered by Suiche who found another domain link hidden in the code.
The NHS, which was showcased in the opening ceremony of the 2012 Olympic Games, employs more than 1.5 million people, making it the world's fifth biggest employer after the US and Chinese militaries, Walmart and McDonald's.
While some practices were not directly attacked by the cyber-attack, some network connections to data servers and the Internet were affected.
Optics had received no external government funding for IT connectivity or infrastructure, but was expected to adhere to the same information governance standards as the rest of the NHS, Dr Hampson added.
"There's a lot of older Windows products out there that are "end of life" and nobody's bothered to take them out of service", said Cynthia Larose, a cybersecurity expert at the law firm of Mintz Levin.
Yesterday's attack's success was likely to be because some hospital trusts and other organisations have either not applied the patch released by Microsoft, or they are using outdated operating systems.
At work, check with your IT administrator to make sure your organization's devices are protected from WannaCry.
"Malware that penetrates the perimeter and then spreads inside the network tends to be quite successful", said Johannes Ullrich, director of the Internet Storm Center at the SANS Institute.
Vicente Diaz, a security analyst at Kaspersky, said big companies would have spent the weekend implementing such measures, but smaller firms without a dedicated security team could still suffer from the malware.
The UK's leading eye health service provider has so far escaped unscathed from a cyber attack that brought large sections of the NHS to a halt last week. But they could still linger as low-grade infections that flare up from time to time.
"Ransomware is following the same trajectory as phishing", said Phillip Hallam-Baker, an expert at the digital security firm Comodo.
BBC analysis of three accounts linked with the global attack suggests the hackers have already been paid the equivalent of £22,080.
"Wannacry" locks users out of their computers and demands hundreds of dollars from victims hoping to regain control of their documents and data.
The damage might have been temporarily contained.
Had it not been for a young cybersecurity researcher's accidental discovery of a so-called "kill switch", the malicious software likely would have spread much farther and faster.
"Our security protection methods have prevented significant impact of this attack within our organisation". "Talk about a wake-up call", Hypponen said.
They should do this before they open their emails or other programs.
"Whenever there is a new patch, there is a risk in applying the patch and a risk in not applying the patch", Grobman said.
It issued guidance for people to protect their systems, while taking the highly unusual step of reissuing security patches first made available in March for Windows XP and other older versions of its operating system.
"We don't want to be alarmists because if you tell people that the sky is falling every day, pretty soon they won't care that the sky is falling". And those fixes will do nothing for newer systems if they aren't installed.
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