David Cameron has criticised a Tory MEP who went on American TV to attack the National Health Service, dismissing his views as "eccentric".
The Conservative leader said the NHS was a "great national institution" and that it was his party's "number one mission" to improve it.
"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."
Mr Cameron brushed aside an attack on the NHS's record by Tory MEP Daniel Hannan, who told American television viewers that he "wouldn't wish it on anybody".
"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," the Tory leader said.
However, Labour was quick to exploit Mr Hannan's remarks, with Business Secretary Lord Mandelson claiming that they had exposed the "two faces" of the Conservative Party.
"I think people will find it shocking that a Conservative parliamentarian should go to the US in order to slag off the NHS, which is one of our great achievements in this country," he told Sky News.
"What we see is the two faces of the Conservative Party - the one David Cameron wants to everyone to see and believe, and the other one presented by the Conservative parliamentarian."
Health Secretary Andy Burnham said Mr Hannan's intervention was Mr Cameron's "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.