Patient data appears not to have been accessed and dumped online as a result of the NHS cyber attack, Amber Rudd has said.
Speaking to ITV News, the home secretary said that "45 different parts of the NHS" had been affected by the ransomware attack, but insisted the UK's response to the incident had been "good".
And Prime Minister Theresa May praised NHS staff for working overnight to ensure records had not been affected.
Quoting Europol, Mrs May said Friday's attack, which has affected over 70 countries worldwide, was "unprecedented" in scale.
Ms Rudd said: "Patients' data has not been released, that of course is people's largest concern.
"And most of them [the 45 NHS divisions affected] are managing to continue, despite this disruption, with their normal course of business."
The Home Secretary plans to chair a Cobra meeting in Whitehall on Saturday.
- Video report by ITV News correspondent Richard Pallot
Ms Rudd said: "We are making sure that the parts of the NHS that have been impacted have the support they need to make sure that patient data hasn't been accessed, and the reassurance we've received from them on the front line is that that is currently the state of play."
But despite the home secretary's assurances, patient data remains inaccessible in many instances and the health service faces a weekend of chaos as IT experts try to restore vital computer systems.
Ms Rudd stressed the malware assault on the NHS had been part of a major international attack and insisted that Britain was at the forefront of tackling such attacks.
"In fact the response has been good in the UK, and I hope we will be able to continue to disrupt it," she said.
The malware programme that has caused such chaos for the NHS and affected businesses in more than 70 countries exploits a security loophole in Microsoft XP software.
Asked why the government allowed the NHS to operate software that is no longer supported by Microsoft, Ms Rudd said that Jeremy Hunt, the secretary of state for health, had been "clear" that the NHS had been asked to upgrade its software.
Mr Hunt has found himself under fire as a result of the cyber attack, with shadow health secretary Jonathan Ashworth accusing him of ignoring "extensive warning signs" of the health service's vulnerability.
In a letter to Mr Hunt on Saturday Mr Ashworth said there had been repeated warnings about the outdated NHS systems, including from the National Cyber Security Centre (NCSC) and the National Crime Agency (NCA).
"NHS Trusts have been running thousands of outdated and unsupported Windows XP machines despite the government ending its annual £5.5 million deal with Microsoft, which provided ongoing security support for Windows XP, in May 2015," Mr Ashworth wrote.
"It effectively means that unless individual trusts were willing to pay Microsoft for an extended support deal, since May 2015 their operating systems have been extremely vulnerable to being hacked," he added.
He said a Freedom of Information Request in February had found a total of 79 English NHS trusts have suffered ransomware attacks since June 2015.