Sir Keir Starmer was locked in a trial of strength with union chiefs demanding a U-turn over limiting winter fuel payments for pensioners.
Unite general secretary Sharon Graham directly told the Prime Minister that he should reverse the controversial policy.
But Sir Keir was showing no signs of backing down.
Downing Street said there were no plans to water down the restriction to the special payments.
There was no discussion of softening the impact of the winter fuel cut at Cabinet on Monday, No10 said after a Home Office minister suggested the policy may be tweaked.
Asked whether Dame Diana Johnson had been "freelancing" when she said mitigations were being looked at by ministers, a No 10 spokeswoman said: "I think it was promptly corrected."
She added: "The Prime Minister has always been clear that he will always be straight with the public about the tough decisions that this Government has to take.”
Asked whether Sir Keir was worried that pensioners may die as a result of the cuts, the spokeswoman said "the focus of Government is ensuring that support is properly targeted to those that need it".
Whipping for the vote over the Government's plans to scrap the winter fuel allowance for all but the country's poorest pensioners was also not discussed at Cabinet, Downing Street said.
From this autumn, older people in England and Wales not on pension credit or other means-tested benefits will not get winter fuel payments, worth between £100 and £300.
Chancellor Rachel Reeves insists the measure is needed as part of plans to plug an alleged £22 billion black hole in the public finances which Labour claim it was left by the Tories.
As dozens of Labour MPs, including some in London, were mulling refusing to back the Government in a vote on Tuesday on the winter fuel payment restrictions, union chiefs piled pressure on the Prime Minister.
Ms Graham, who heads Britain’s biggest union, bluntly told Sir Keir to “do a U-turn” on the policy, which would see winter fuel payments restricted to only the poorest pensioners.
She told the BBC’s Today programme: “We need to make sure that he is making the right choices and leadership is about choices. He needs to be big enough and brave enough to do a U-turn on this choice. It’s completely wrong.”
She called instead for a tax on the “1% wealthiest”, which she claimed would raise £25 billion rather than the £1.2 billion restricting winter fuel payments would save.
She said: “The wealthiest in the country have been at the front of the queue for 14 years. I want to see workers and communities now at the front of the queue and that is why we’re saying they must do a U-turn on this very, very, very cruel (policy).”
She claimed that the Government had decided to “pick the pocket of pensioners” while leaving the richest “totally untouched”, despite the Chancellor having signalled tax hikes are coming in the autumn Budget.
Fran Heathcote, general secretary of the Public and Commercial Services Union, crriticised the winter fuel move as a “misstep” which needed to be “put right”.
With the threat of further spending cuts, she told the BBC: “I can see a situation where, if they continue along the line that they’re heading, not just winter fuel payments but with social security and benefits more generally, there will be a real backlash and that could take the form of industrial action... because lots of unions represent low paid workers.”
Paul Nowak, general secretary of the Trades Union Congress, was slightly less hardline than other union chiefs.
He told BBC Breakfast: “I’ve got real concerns about the cuts to winter fuel allowance because I don’t want any pensioner going into this winter worried about putting the heating on.
“That’s why I hope in the Budget the Chancellor will set out the support that she’ll make available to those pensioners who ... aren’t well off by any means. To make sure that they’re not frightened to use the heating this winter.
“But I think it’s fair to say the Chancellor’s got a huge range of challenges. She’s been bequeathed a toxic economic legacy by the previous government. There’s lots of things that she needs to fix. The state of our public services, the fact that we’ve got a universal credit system that’s not fit for purpose.”
Policing minister Dame Diana Johnson, who clashed with Good Morning Britain presenter Susanna Reid over the controversial policy, gave no sign that the Government was set to retreat from its changes on winter fuel payments.
“Across government, the Chancellor has identified a £22 billion black hole. Really difficult decisions are going to have to be made,” she told LBC Radio.
“This is really difficult. Nobody wants to do it, but this is one of the policies that the Chancellor has identified that we need to act on.
“The budget is obviously coming in October. We also have committed to the triple lock on pensions to protect pensioners income. We want to make sure that everybody who’s entitled to pension credit, claims it, because that will be a winter fuel allowance passport through, if they get the pension credit. And we want to do more on insulation of homes, because we know that’s a really big issue for many older people.”
Treasury sources said Dame Diana “misspoke” in suggesting the Government was looking at doing more than encouraging further take-up of pension credit.
