Gordon Brown has claimed the Tories are "testing the water for a different kind of NHS " as he issued a warning against a two-tier health system.
The ex-Labour Prime Minister made the claim after the senior Conservative MP Sajid Javid raised the prospect of charging for GP appointments.
It also comes amid a major crisis facing the NHS with a record backlog, strike action among tens of thousands of staff, and record waits for A&E.
In an article, Mr Brown accused the Tories of abandoning the value "healthcare is not a privilege to be bought, but a right for all who are in need of it".
He also raised Rishi Sunak's pledge during last summer's Tory leadership campaign to charge patients £10 for missed GP appointments.
At the time Mr Sunak said the measure would help tackle the record backlog - but ditched the policy after winning the keys to No10 in October.
Raising the Prime Minister's proposal, Mr Brown warned in The Guardian: "Conservatives are testing the water for a different kind of NHS".
He claimed the direction the party is travelling "is already clear", adding: "The sick would pay for being sick and charging would force, as has happened with GP and hospitals in France, the better-off sections of the population to take out private insurance - inevitably creating, in its wake, a two-tier healthcare system."
"Today’s Conservatives may have clapped NHS nurses and health workers at the height of the pandemic; yet they are not only opposing decent remuneration for them, but also contemplating a more privately financed healthcare system.
"It reminds us that exactly 75 years ago they opposed the introduction of the NHS, having attempted in 1944 to impose charges and private insurance."
At the weekend, Mr Javid, who served as Health Secretary between June 2021 and July 2022, described the health service's current model as "unsustainable".
Mr Javid, who has also announced he will resign at the next election, wrote in The Times the "introduction of a contributory principle... will be crucial" - but also acknowledged the "conversation will not be easy".
The former Cabinet Minister highlighted Norway and Sweden as examples where a visit to the GP "comes with a contribution of about £20".
In response, Labour's Shadow Health Secretary Wes Streeting said: "Over my dead body.
"An NHS free at the point of use has been its central equitable principle for 75 years. Patients should never have to worry about the bill.
"It’s up to Labour, which founded the NHS, to grip the biggest crisis in its history and make it fit for the future."
Asked about Sajid Javid's proposal, the Prime Minister's official spokesman said there were "no plans" to introduce fees for GP appointments.
