More than two out of five people do not trust the government to protect the NHS, according to a study.
A further one in four said their trust had fallen over the past year, a survey of more than 2,000 adults for campaign group 38 Degrees showed.
The research was published ahead of a protest on August 30 against a trade deal being negotiated between the European Union and the United States.
Unions and campaign groups claim the deal, the Transatlantic Trade and Investment Partnership, would lead to health services being opened up to competition from US private healthcare providers.
Blanche Jones, campaign director of 38 Degrees, said: “It’s no surprise that the public is worried about the fate of their local GPs and hospitals. Our NHS is being used as a bargaining chip in a deal done behind closed doors.
“Most of us only need to take one look at American healthcare to know that that isn’t what we want in this country.
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“That’s why thousands of 38 Degrees members will be taking to the streets this Saturday to spread the word about this new threat to our NHS.
“This will be a bad deal for ordinary people. Politicians of all parties now need to explain exactly what they’re doing to get it fixed or scrapped.”
The government has denied the claims from campaigners, insisting the trade deal would generate new jobs and be a boost for business.
Unite general secretary Len McCluskey said: “This new poll from 38 Degrees piles even more pressure on David Cameron to act and exempt the NHS from [the partnership].
“David Cameron’s silence is deafening. He is refusing to answer a very simple question: are we going to exempt health from the EU US trade agreement?
“The people of this country didn’t vote for selling off our NHS and they didn’t vote to make the sell-off irreversible by giving US companies the right to sue us in secret courts if the government tries to reverse privatisation.”