The prize-winning physicist announced on Friday that he was putting his weight behind the JR4NHS campaign to stop Health Secretary Jeremy Hunt getting private companies to run parts of the health and social services.
Founded by three doctors and a university professor, JR4NHS says that the introduction of accountable care organisations (ACOs) will “americanise” the NHS.
JR4NHS filed judicial review papers on Monday in a demand for a public consultation and parliamentary scrutiny before changes are introduced.
Campaigners fear that the government is trying to sneak through a switch to ACO systems “during the clamour of Brexit.”
Prof Hawking said in a statement: “I have been lucky to receive first-rate care from the NHS. It is a national institution, cherished by me and millions of others and which belongs to all of us.
“I am joining this legal action because the NHS is being taken in a direction which I oppose, as I stated in August, without proper public and parliamentary scrutiny, consultation and debate.
“I am concerned that ACOs are an attack on the fundamental principles of the NHS.”
On its page on Crowdjustice.com, JR4NHS says that ACOs will decide the boundary of what care is free and what has to be paid for and that the ACOs will be paid more if they “save” money.
A Department of Health spokesman said that the warnings from the campaign group were “irresponsible scaremongering.”
He added: “The NHS will remain a taxpayer-funded system free at the point of use. ACOs are simply about making care more joined-up between different health and care organisations.
“Our consultation on changes to support ACOs is entirely appropriate and lawful.”