Harriet Harman faces embarrassment tonight (20 July) when tens of Labour MPs are expected to defy the party's whips and vote against George Osborne's welfare reforms.
The acting Labour leader had originally ordered the MPs to abstain from voting on the draft legislation.
But after an internal Labour uproar, which saw leadership hopefuls Andy Burnham, Yvette Cooper and Jeremy Corbyn rail against her plan, Harman was pushed into tabling an amendment to the Conservative's welfare bill.
The "reasoned amendment" sets out what Labour supports and what the party opposes, including the government's plan to scrap the current child poverty targets.
However, Helen Goodman has put forward her own amendment, which the Bishop Auckland MP says has gained at least 60 signatures.
The issue has enraged the left-wing of Labour and threatens to undermine the party's unity until Ed Miliband's successor is announced.
John McDonnell, chair of Labour's Socialist Campaign Group, told IBTimes UK on 13 July that he would oppose Osborne's welfare bill.
"I want to make it absolutely clear that I will vote against the government's welfare cuts. I will not vote for policies that impoverish more of our families and children," the Hayes and Harlington MP said.
The chancellor, meanwhile, has urged "moderate" Labour MPs to back his welfare reforms in an opinion piece for The Guardian.
I urge moderate Labour MPs not to make the same mistake as in the last parliament, when they refused to support each and every welfare reform we proposed. I say: vote with us," Osborne wrote.
"Welfare reform is not just about saving money. It's about transforming lives, and social justice. This is the new centre of our politics. All those who call themselves progressives should join us."
The SNP have lodge their own amendment against the welfare bill, specifically attacking the Conservative's plans to cut tax credits.
"Labour may not be willing to stand up to the Tories and oppose these cuts which leave 13 million families worse off – but the SNP will provide the real opposition in this parliament," said Dr Eilidh Whiteford MP, the SNP's welfare spokesperson.
"In Scotland tax credits are overwhelmingly paid to working people. And in Scotland 95% of tax credits are paid to families with children. So we should make no mistake about where the cuts are being targeted. And the idea that the changes to the minimum wage will fill the gap is simply ludicrous as every sensible economic commentator has made clear."