Anas Sarwar has criticised the prime minister's decision not to compensate the Waspi women affected by the UK governments failure to warn them about changes to the state pension.
The Scottish Labour leader described the decision as "deeply disappointing" and insisted a "different way could have been found".
Despite signing a pledge drawn up by Scottish campaigners for "fair and fast compensation" in 2022, Sir Keir Starmer said the Treasury couldn't afford to pay out compensation.
The issue is the first rift between the political heads as Mr Sarwar dismissed the argument by Liz Kendall, the UK work and pensions secretary, that a compensation package was unaffordable.
She announced on Tuesday there would be no compensation for women born in the 1950s who were not aware of changes to the state pension age. She later described it as an "extremely difficult decision".
Women Against State Pension Inequality (Waspi) says 3.6 million women born in the 1950s - including hundreds of thousands in Scotland - were not properly informed of the rise in state pension age to bring them into line with men.
Mr Sarwar, who has for years supported the Waspi campaign, hit out at Labour ministers for not agreeing to pay compensation.
He said: “I don’t think the decision the government has come to is the right one on compensation. I think they’ve come to the right place on injustice and they’ve come to the right place on an apology, but I don’t think it’s the right decision on compensation.
“I think, given the public finances, I think a different way forward could have been found.”
Mr Sarwar added: “I agree with them on their frustration and that’s why I campaigned alongside them and I accept that frustration.
“Therefore, as I say, I don’t think this is the adequate package. I think it was right to have the apology, I think it was right to respect the injustice, but I think a fairer decision could have been made around the compensation.
“That fairer decision could have been recognising the current financial situation. They could have looked at targeted support for lower-income pensioners, they could have looked at tapering, they could have looked at increments. They could have looked at a whole host of issues.”
The attack on the prime minister comes after Sarwar's allies fear his chances of becoming first minister after the 2026 Holyrood election have been damaged after the early months of Mr Starmer's government.