Ed Miliband is under increasing pressure from within his own party ahead of a key Commons vote on Europe already threatening to tear apart the Conservatives.
A number of Labour MPs are understood to be considering voting with Tory eurosceptics on a crunch amendment.
Three Labour MPs have already offered support for the motion, which laments the omission of an EU referendum in last week's Queen's Speech.
Yesterday, David Cameron received unexpected support from Barack Obama ahead of tomorrow's expected showdown when more than 100 Tory MPs are expected to vote against their party leadership.
In Washington, the US President cautioned the Conservative rebels to trust their leader and backed the Prime Minister's plan to renegotiate the UK's relationship with Europe ahead of an in-out EU referendum.
"You probably want to see if you can fix what is broken in a very important relationship before you break it off," he said.
But he also sounded a more cautious note, describing the UK's EU membership as an expression of its global influence.
For his part, Mr Cameron stepped up his attacks on the dissenters within his own party, just days after he denounced them as "pessimists".
He accused the rebels of "throwing in the towel" before negotiations had even opened.
Education Secretary Michael Gove and Defence Secretary Phillip Hammond both raised eyebrows at the weekend when they became the first Coalition ministers to say they would vote to leave the UK if a referendum was held today.
Labour accused the Tories of tearing themselves apart over Europe again, with one source expressing amazement at Mr Gove and Mr Hammond's comments.
"Which minister answers hypothetical questions?" he asked.
But Labour is also facing increasing pressure on the issue.
At the weekend, Mr Miliband refused to pledge to hold an in-out referendum, warning it would lead to four years of uncertainty and potentially destabilise the economy.
But a group of Labour MPs have launched a campaign to convince him to back a public vote.
Labour MPs who support the campaign are thought to include former minister Frank Field and chairman of the Scottish Affairs Committee Ian Davidson.
They believe the public deserves a say on EU membership, calling on Labour to set a clear position on the issue.
The potential for a referendum bill in the Queen's Speech was always a non-starter as it was opposed by the LibDem half of the Coalition.
Many of those who back the rebel amendment are keen to force both the LibDems and Labour to vote against the move.
They argue this would allow the Conservatives to go into the next General Election as the only party promising an EU referendum.
But the result is expected to be a significant blow to Mr Cameron's authority just weeks after his creation of a new No 10 policy team had appeared to temporarily silence the doubters within his own party.
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