David Cameron has mounted a damage limitation exercise after a Tory MEP launched an outspoken attack on the NHS.
The Conservative leader dismissed Daniel Hannan's views - aired on American TV - as "eccentric" and insisted delivering improvements to the NHS was his party's "number one priority".
But Labour was quick to seize on Mr Hannan's comments, claiming they exposed the Tories' "deep ambivalence" towards the health service.
Mr Cameron declared 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."
He brushed aside the remarks by Mr Hannan, who told US television viewers that he "wouldn't wish it on anybody". Mr Hannan - a political maverick who has long argued for the NHS to be dismantled - had been taking part in a series of US television debates on President Barack Obama's plans to reform the American healthcare system.
"He does have some quite eccentric views about some things, and political parties always include some people who don't toe the party line on one issue or another issue," said Mr Cameron. It was the second time in as many days he has been forced to slap 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.
Health Secretary Andy Burnham said the Tory leader was "rattled" and Mr Hannan's intervention had been his "worst nightmare".
"What has happened within the last 48 hours is what Cameron has feared most because it lays bare the Tories' deep ambivalence towards the NHS," he said. "Cameron knows there is deep hostility towards it within his ranks. Hannan is not the only one - many senior Tory MPs would privately agree with his comments."
Business Secretary Lord Mandelson said the public would be shocked to see a Conservative politician flying out to the United States to "slag off" the NHS. "What we see is the two faces of the Conservative Party - the one David Cameron wants everyone to see and believe, and the other one presented by the Conservative parliamentarian," he said.