The Chancellor George Osborne failed to show how the Tories would meet their pledge to pump an additional £8bn ($11.6bn) into the NHS by 2020 in a tense TV interview.
The senior Conservative and election strategist repeatedly refused to explain the maths behind his party's flagship health policy on the BBC's Andrew Marr show.
Osborne side-stepped Marr's persistent questioning on the proposal and claimed the election commitment was part of a "balanced plan", while failing to offer any specifics.
He did, however, point out that NHS funding has been raised over the lifetime of the Coalition Government.
"As Tory election strategist, you've just blown a huge hole in your own strategy by making an £8bn unfunded pledge on the NHS – it's exactly what you said you were not going to do," Marr said.
Osborne replied: "I don't accept that at all. We have a balanced plan to grow our economy, to make savings in government, including in welfare, to fund our NHS, each and every year, to give a tax cut to working people so we make work pay and we support public services. We have an economic policy that supports our entire country."
Labour seized on the comments and stressed that "you can't fund the NHS on an I.O.U."
"We have seen five years of failure and broken promises from David Cameron on the NHS and now he expects the public to believe unfunded promises four weeks before an election," Ed Miliband said.
"Just in January he said and I quote "the real risk to the NHS was the risk of unfunded spending commitments bringing chaos to our economy, which would wreck our NHS."
He added: "The truth is you can't save the NHS if you don't know where the money is coming from.
"You can only damage the NHS when they are planning colossal cuts in public spending year after year after year."