David Cameron is engaged in a damage limitation exercise after two of his MEPs attacked the NHS.
The Tory leader did not mince his words in dismissing Daniel Hannan's negative views about the health service as "eccentric".
But he then suffered fresh embarrassment when one of Mr Hannan's Conservative colleagues in the European parliament expressed support for his position.
Interviewed on BBC Radio 4's PM programme, Roger Helmer insisted: "Now we all love the NHS, but I think we all know in our hearts that it is no longer the envy of the world."
He went on: "If the Americans came to me and said would you recommend us taking up a system just like the British NHS? I think I would have to say 'No'."
Labour has been quick to capitalise on Mr Cameron's discomfort, insisting the affair exposed "deep ambivalence" within his party towards the health service.
However, Mr Cameron stressed he was fully committed to the NHS, which he described as a "great national institution".
"The Conservative Party stands four square behind the NHS," he told reporters in his Oxfordshire constituency.
"We are the party of the NHS, we back it, we are going to expand it, we have ring-fenced it and said that it will get more money under a Conservative government, and it is our number one mission to improve it."
It was the second time in as many days he has slapped down a member of his own party, having already rebuked senior frontbencher Alan Duncan over his claim that MPs were having to survive on rations in the wake of the expenses scandal.
