Stephen Hawking has said he is worried about the future of the NHS, in a speech critical of government policy and Health Secretary Jeremy Hunt.
The Cambridge University scientist, who publicly backed Labour in the election, accused Mr Hunt of "cherry-picking" evidence to support his policies.
The 75-year-old also said he was concerned about the involvement of the private sector in the NHS in England.
But Mr Hunt said some of Mr Hawking's comments were "pernicious".
A statement released by the Department of Health after the text of the speech was given to the BBC in advance said extra money was being invested in the NHS and it had recently been ranked as a top-performing health system.
Prof Hawking, who has had motor neurone disease for most of his adult life that has impaired his movement and ability to speak, delivered the speech at a conference at the Royal Society of Medicine in London, organised to air concerns about the future of the NHS.
The author of A Brief History of Time, who is a Labour supporter, said he had been motivated to speak because of the role the health service has played in his life, saying if it was not for the NHS he "wouldn't be here today".
How the NHS has helped Prof Hawking
In the speech, Prof Hawking listed a number of occasions on which the NHS was there for him.
This included an episode in 1985 when he caught pneumonia in Switzerland.
Doctors there suggested his ventilator be turned off to end his life, but his wife refused and he was flown back to Addenbrooke's hospital in Cambridge where he received treatment and recovered.
Fourteen years after that, he had pioneering throat reconstruction surgery in London after his condition worsened and he was struggling to eat and breathe.
"I have had a lot of experience of the NHS and the care I received has enabled me to live my life as I want and to contribute to major advances in our understanding of the universe," he said, referring to his theories on black holes and the origin of the Universe.
Why Prof Hawking is worried about the NHS
His speech then listed some of the developments in the NHS that concerned him, including the move toward what he called a "US-style insurance system".
He said he believed there had been an increase in private provision of care, including the use of agency staff, that was leading to profit being extracted from the health service.
"The more profit is extracted from the system, the more private monopolies grow and the more expensive healthcare becomes. The NHS must be preserved from commercial interests and protected from those who want to privatise it," he said.
He said that a publicly provided, publicly run system was the "most efficient" and so those who say we cannot afford the NHS are wrong.
"We cannot afford not to have the NHS," he added.
His speech also mentioned Mr Hunt by name.
In a section about the move towards a seven-day NHS, Prof Hawking said that while he would like there to be more services available at weekends, the government has failed to carry out "proper due diligence", particularly with regard to whether there would be enough staff.
He quoted from a letter he put his name to last year explaining how Mr Hunt "cherry-picked" research to put his case.
What has Jeremy Hunt said?
The health secretary initially said on Twitter that Mr Hawking was a "brilliant physicist but wrong on the lack of a weekend effect" in the NHS.
He also said the the study into mortality rates associated with weekend NHS services was "the most comprehensive ever".
But hours later he posted two further tweets, in which he said Mr Hawking's concerns, about the development in the UK of the type of insurance system seen in the United States, were a "pernicious falsehood".
He also said the Conservatives had provided the NHS with more money and medical staff than ever before.
Most pernicious falsehood from Stephen Hawking is idea govt wants US-style insurance system.Is it 2 much to ask him to look at evidence? 1/2— Jeremy Hunt (@Jeremy_Hunt) August 19, 2017
NHS under Cons has seen more money,more docs and more nurses than ever in history.Those with private med insurance DOWN 9.4% since 2009! 2/2— Jeremy Hunt (@Jeremy_Hunt) August 19, 2017
The government's defence
The Department of Health responded to Mr Hawking's comments by pointing out that the numbers of staff working in the NHS were increasing and it "makes no apology" for tackling the weekend effect.
The statement pointed out that only about 8% of NHS funding goes to the private sector.
It also said that "despite being busy", the NHS had been ranked as the "best, safest and most affordable healthcare system out of 11 wealthy nations" in a recent study by the Commonwealth Fund.
"The government is fully committed to a world-class NHS, free at the point of use now and in the future - that's why we're backing it with an extra £8bn of investment over the next five years," the statement said.