ED Miliband could face one of the biggest rebellions of his leadership today amid growing pressure over Labour's lacklustre showing in the polls.
The test will come as Labour backs a Coalition Government move to cap the welfare bill.
Around 20 Labour MPs are expected to rebel and vote against the limit, although it is understood that figure could grow.
Conservative MPs cheered yesterday when the party confirmed it would vote with the Government, despite criticism from its own backbenchers.
The Tories sprung the vote on Labour at last week's Budget and hope it will show voters which party can be trusted to make the public spending cuts necessary after the next election.
The row follows pressure on the Labour leader from his own MPs. They are increasingly nervous about opinion polls which suggest Labour has lost its lead over the Tories. Mr Miliband is also considered by many within his party to have performed badly after last week's Budget statement.
Yesterday, another Labour MP admitted that Mr Miliband had yet to convince voters he was ready to lead the country. Former minister David Lammy also suggested that the party had become too concerned with its cost of living message instead of a "positive offer" to voters.
Former Labour chairwoman Hazel Blears had called on the party to make "faster progress" on policy and demanded the party address the problems of voters in "normal human language".
At the start of the week a group of influential thinkers called on Mr Miliband to produce "transformative" policies that would excite voters. They warned the Labour leader not to rely on Tory unpopularity to secure victory.
But Labour's deputy leader Harriet Harman hit back, saying that the party was putting forward "radical and sensible" ideas.
"I think we are making steady progress," she said. "Yes, it is a big task and we're not complacent, but I think that we are addressing the issues that people are concerned about."
Last night, Rachel Reeves, Labour's shadow work and pensions secretary, confirmed the party would vote for the welfare cap. But she said her party would also tackle "the causes of rising social security spending; low pay, long-term unemployment and the inadequate supply of housing".