Gordon Brown has stepped up his attack on David Cameron over the NHS row, branding the Tories "two faced".
The Prime Minister insisted that criticism of the health service by two Conservative MEPs had revealed the "truth" about the party's attitude.
Lord Mandelson also waded in by bluntly accusing Mr Cameron of being a "fraud" who said one thing in public, and another privately.
The Tory leader has been engaged in desperate damage limitation since Daniel Hannan went on US television to warn against copying the British system, saying he "would not wish it on anybody".
Mr Cameron dismissed the remarks as "eccentric", claiming that his party stood "foursquare behind the NHS" and was committed to real-terms funding increases.
But there was further embarrassment when one of Mr Hannan's Conservative colleagues in the European parliament, Roger Helmer, expressed support for his position.
The interventions came in the context of a major row in America sparked by President Barack Obama's proposals for healthcare reform. Republicans claimed the changes would create a similar system to the NHS, which they condemned as "Orwellian" and putting the power of life and death into the hands of a bureaucracy.
The ferocious attacks led to a backlash on this side of the Atlantic, with thousands signing up to a #welovetheNHS campaign on the Twitter social networking site.
Mr Brown sent a letter to Labour Party members on Saturday night praising their support for the health service.
He wrote: "It is understandable that the Conservative leadership have tried to distance themselves from those in Tory ranks who criticise the NHS. But the truth is that there are two Tory faces on the NHS. Behind all the recent talk of commitment, the party has not truly been reformed."
