Ed Miliband is appealing to Liberal Democrat peers to join with Labour to wreck the Government's controversial NHS reforms.
The Health and Social Care Bill is set to resume its troubled parliamentary passage in the House of Lords, amid growing calls for a major overhaul.
Despite widespread opposition within the medical profession, David Cameron has given his full backing to Health Secretary Andrew Lansley - the plan's architect.
However, Lib Dem peers have tabled a series of detailed amendments while the party's president Tim Farron is urging them to strip out all new competition elements from the legislation.
Writing in the Sunday Mirror, Mr Miliband said Lib Dems should now combine with Labour in the upper chamber to deal a death blow to the bill.
With the Lib Dem leadership facing a potential grassroots revolt over the issue at their spring conference next month, the Labour leader warned they could suffer an even bigger backlash than they did over student tuition fees.
"This week is the time for everybody to stand up and be counted. The Liberal Democrats in the House of Lords must join with Labour to hole David Cameron's health plans below the water line," Mr Miliband said.
Amid widespread criticisms that Mr Lansley has failed to explain his reform programme to the public, a Cabinet minister acknowledged they had to do more to make the case.
"All of us, not just Andrew, all of us have to do better in explaining why these reforms are taking place," International Development Secretary Andrew Mitchell told BBC's The Andrew Marr Show.
"These reforms stem from an absolute and total commitment to the NHS."