Keir Starmer is under increasing pressure to scrap the controversial two child benefit cap after the SNP announced it will abolish the limit in Scotland.
The move would lift 15,000 youngsters out of poverty, Scottish finance secretary Shona Robison said.
Labour has so far stuck with the cap, one of George Osborne’s ‘austerity’ measures, but pledged to ‘do the sums’ on how much abolition would cost.
But the policy, hated by many within Labour, triggered the first rebellion of his premiership earlier this year.
Charities condemn the policy, which affects 1.6 million children across the UK, as “cruel”.
The SNP administration in Scotland has declared eradicating child poverty to be its top priority.
Hitting out at Sir Keir, Ms Robison said many had looked to Labour to end the "pernicious" two child limit - but that the Scottish Government will act where Westminster had not.
Outlining her tax and spending plans in Holyrood on Wednesday, Mr Robison said: "Be in no doubt that the cap will be scrapped."
Scottish ministers would "work as hard as possible" so payments can start being made "as early as we can in 2026", she said, adding that the move would require the co-operation of the UK Government, which has the relevant data on the families which lose out.
The change is forecast to cost around £150 million in 2026-27, rising to more than £200 million by 2029-30.
Throwing down the gauntlet to the prime minister, Ms Robison challenged Labour to "join us in ending the cap in Scotland", but she told MSPs: "Let me be crystal clear, this government is to end the two-child cap and in doing so will lift over 15,000 Scottish children out of poverty."
In another challenge to Sir Keir, the Scottish Government has already promised to reintroduce winter fuel payments for all pensioners north of the border from next year.
The charity Child Poverty Action Group said the Scottish Government had made the "right decision", with chief executive Alison Garnham adding: "Westminster must now step-up and scrap the two-child limit UK-wide. There can be no justification now for Westminster dragging its feet and continuing to roll out poverty to more and more children through this policy from the austerity era."
Chris Birt, association director for Scotland at the Joseph Rowntree Foundation, said the Scottish Budget shows "positive signs of investing in the right things".
He added: "Committing to the eliminating of the two-child limit by investing in social security is a positive recognition that political choices can reduce poverty.”
