Shadow health secretary Andy Burnham has written to all Liberal Democrat MPs urging them to vote against the coalition Government's plans for reform of the NHS.
Mr Burnham also challenged Lib Dem and Conservative health spokesmen to a public debate with him on the reforms, which he will describe in a speech on Tuesday as the biggest threat to the NHS in its 62-year history.
The Labour leadership contender warned that plans to give GPs control of an estimated £80 billion of NHS spending and to hand more independence to hospital trusts will destabilise the service, throwing it into organisational chaos at a time when it needs stability.
His intervention comes days after the health union Unison launched a legal challenge to the reforms on the grounds that a consultation process being conducted by Health Secretary Andrew Lansley is a sham.
In his letter on Monday, Mr Burnham said Liberal Democrat voters were not expressing support for reforms of the kind set out in Mr Lansley's health White Paper when they cast their ballots in the general election in May.
He told Lib Dem MPs: "You hold the key to the future of our NHS. People who voted for you at the election did not vote for such a radical break-up plan. I urge you to listen to them and stand up for our NHS in the face of this attack, which threatens to unpick its very fabric."
In a speech in Liverpool on Tuesday - shortly before the Lib Dems' annual conference in the city next month - Mr Burnham is expected to say: "There are former Lib Dem councillors here in Liverpool who don't know that their party stands for any more.
"When Nick Clegg comes here the week after next, he must explain why these reforms are being forced on the NHS in direct contradiction to what the coalition agreement promised."
Mr Burnham will argue that introducing massive structural reform at a time of financial retrenchment will "throw the NHS into chaos" at a time when it needs stability.
Handing commissioning power to GPs risks a postcode lottery, with different standards and accessibility of care depending where patients live, he will say.