Theresa May has raised fears the NHS is "for sale" after refusing to rule out allowing US firms to compete for health service contracts after Brexit.
Critics accused the Prime Minister of giving Donald Trump the "green light to get his hands on" the NHS after she sidestepped calls to exclude it from trade talks.
Liberal Democrat leader Sir Vince Cable, who quizzed Mrs May on the issue during Prime Minister's Questions, said her response had been "pathetic".
Mr Trump described the NHS as "going broke and not working" in a tweet on Monday.
But Mrs May failed to raise the UK President's attack on the health service when they spoke by telephone on Tuesday.
During PMQs, Sir Vince Cable asked Mrs May to confirm that in her conversations with President Trump she had made it "absolutely clear" the "NHS is not for sale".
"The Prime Minister knows that one of the key objectives of American trade negotiators in any future deal after Brexit is to secure access for American companies to business in the NHS," he said.
"Can she give an absolute guarantee that in those negotiations the NHS will be excluded from their scope, and can she confirm that in her conversations with President Trump she's made it absolutely clear to him that the NHS is not for sale?"
Mrs May replied: "We are starting the discussions with the American administration first of all looking at what we can do to increase trade between the US and the United Kingdom already, even before the possibility of any free trade agreement."
Labour MP Peter Kyle, a supporter of the Open Britain campaign against a hard Brexit, said: "Theresa May just gave Donald Trump the green light to get his hands on our National Health Service.
"Just days after the US President took to Twitter to insult the NHS, the Prime Minister was given a clear opportunity to rule out opening up our health service to private competition from US companies.
"Her clear refusal to do so underlines her weakness in trade negotiations and should concern us all."
Green party co-leader Caroline Lucas warned of a "profound" threat to the health service.
She tweeted: "Today in #PMQs the Prime Minister refused to guarantee protection for the #NHS in a US trade deal. Make no mistake, an extreme Brexit is a profound threat to our health service - and MPs who love the NHS need to oppose the Tory Brexit plans."
Asked whether Mrs May would rule out the inclusion of the NHS in any US trade deal, a senior Downing Street source said discussions were still at their outset, but pointed out the Government ensured the health service was excluded from earlier discussions on the failed TTIP trans-Atlantic trade agreement.
"What she said is that we are at the outset of these discussions," said the source. "We are not into the detail of these negotiations.
"When you look at the debate on TTIP, we were very clear from our side that the NHS had specific protections and wouldn't be included."
A spokesman for Labour leader Jeremy Corbyn said it was "a matter of great concern" Mrs May had not felt able to rule out the inclusion of the NHS in a future deal.