Sir Keir Starmer was scrambling to limit the size of a Labour revolt over welfare reforms which was threatening to be the biggest yet of his year-long premiership.
Cabinet ministers were holding talks with potential rebels to try to persuade them to back the Government or at least abstain in a crunch Commons vote on Tuesday evening.
But dozens of Labour MPs were expected to still refuse to back the controversial shake-up of the benefits system, despite a series of concessions having already been made.
A reasoned amendment aimed at halting the progress of the Government's welfare reforms has been selected for separate decision, Speaker Sir Lindsay Hoyle said.
The amendment was tabled by Labour MP Rachael Maskell (York Central) and has been signed by 39 Labour backbenchers, far short of the 83 needed to defeat the Government.
However, Ms Maskell,told MPs: "These Dickensian cuts belong to a different era and a different party. They are far from what this Labour Party is for: a party to protect the poor.
"These are my constituents, my neighbours, my community, my responsibility, and I cannot cross by on the other side."
But Work and Pensions Secretary Liz Kendall said the reforms to the welfare system were needed to ensure its longevity, as she sought to head off concerns from rebels about the number of people it will push into poverty.
She stressed that the current rate of new claimants of personal independence payments (Pip), 1,000 a day, would be the equivalent of adding a city the size of Leicester to the welfare budget every year.
The Cabinet minister told the Commons: "I do not believe that this is sustainable if we want a welfare state that protects people who most need our help for generations to come."
She added: "There is no responsibility in leaving our system of social security to continue as it is, and risk support for it becoming so frayed that it is no longer there to provide a safety net for those who can never work, and who most need our help and support."
But both Conservative MP Paul Holmes (Hamble Valley) and Labour MP Polly Billington (East Thanet) raised an impact assessment showing reforms will put an extra 150,000 people into poverty.
No10 says the figure does not take into account more support to get people into work.
Tory leader Kemi Badenoch said the Government's welfare reforms were "driven not by principle but by panic" and she claimed that tax rises were probably “inevitable” after major concessions costing billions were made to avert a Commons defeat on the controversial changes.
The original proposals were expected to save around £5 billion by 2030, but Ms Kendall revealed on Monday that the revised proposals were likely to save around half that figure.
The rebellion, and how the PM responds to it, will inevitably raise questions over the Prime Minister’s authority.
One option is to remove the whip from rebels but the bigger the revolt, the more difficult it is to take this course of action..
Ministers hope the partial U-turn already announced will be enough to win over enough Labour rebels to get the reforms through their first stage in the Commons.
The concessions included protecting people claiming personal independence payment (Pip) from changes due to come into effect in November 2026, and rowing back plans to cut the health-related element of universal credit.
But backbench anger has continued to simmer, with a statement from Ms Kendall laying out the concessions on Monday receiving a negative response.
Some 126 Labour MPs had previously signed a “reasoned amendment” proposed by Treasury Committee chairwoman Dame Meg Hillier that would have stopped the legislation if approved.
She backed voting for the bill at Second Reading on Tuesday, stressing that major changes had already been made and MPs should still fight to further improve the reforms.
One of the chief concerns revolves around a review of Pip to be carried out by disabilities minister Sir Stephen Timms and “co-produced” with disabled people.
His review is not expected to report until autumn next year, making it difficult to incorporate his findings into the Pip changes due to take place at the same time.
