Ed Miliband has said the Conservatives would slash NHS jobs and lose hospital beds under "swingeing cuts".
The Labour leader said the NHS was under "grave danger" because Cameron had broken promises over the health service.
Figures released by Labour claimed there would be a cash shortage at two thirds of trusts that were predicted to be in deficit by the end in the current financial year.
Making a speech in Bedfordshire, Miliband warned the holes in NHS funding meant trusts faced having to make staff cuts and bed reductions, while entire services could be mothballed.
In contrast, he said Labour would build a "compassionate" NHS.
"Right now, our NHS is in grave danger because David Cameron has broken his promises on the NHS," Miliband said.
"He's closed hospitals he said he would keep open, he's allowed waiting times to rise when he said he would keep them low, and he's wasted £3 billion on a top-down re-organisation which drives forward privatisation that he insisted before the last election would never happen.
"Now he's at it again, promising to protect the NHS with just a flimsy IOU. And today we discover the financial bombshell that he has kept hidden from everyone until now.
"Two thirds of hospitals face having to make swingeing cuts, not some point in the future, but this year because of a cash crisis made in Downing Street.
"That will mean staff cut, beds lost and services closed. And it is why we need Labour's better plan for the NHS: a fully funded plan to get more resources into the NHS and start turning things around; a real plan with real money for real action right now."