Theresa May ridiculed Labour's civil war over the Barnett formula
It came after Scottish and UK party officials distanced themselves from a plan to scrap the funding system for Holyrood and other devolved administrations.
Welsh First Minister Carwyn Jones plunged his party into chaos on Monday when he said the cash mechanism would be ditched if his party win the general election.
Scottish Labour leader Kezia Dugdale insisted the party is standing by the formula.
A UK Labour spokesman also denied such a plan, saying it would consult on long-term reform of funding allocation.
But wading into the row the Prime Minister said: "The infighting that's taking place is between Carwyn Jones, and the Welsh Labour Party, and Jeremy Corbyn's Labour Party.
"They can't even agree on how much money is going to be available to Wales under a Labour government."
SNP deputy leader Angus Robertson said Jeremy Corbyn "must make clear how much a Labour government would cut Scotland's budget by".
Robertson said Corbyn 'must make clear how much a Labour government would cut Scotland's budget'
He added: "This is an astounding admission from one of Labour's most senior figures, which has fatally undermined Kezia Dugdale's attempts to portray Labour as standing up for Scottish interests."
Welsh Labour has repeatedly argued the formula has left the country out of pocket compared to Scotland and Northern Ireland.
Its manifesto states: "We need long term reform of how the UK allocates public expenditure to ensure that it reflects the needs of different parts of our country."
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Scottish Labour leader Kezia Dugdale insisted the party is standing by the formula
But Ms Dugdale said the UK party manifesto supports the formula and had pledged an extra £3 billion in so called so-called Barnett consequentials because of Mr Corbyn's planned public spending spree.
She added: "I spoke to Carwyn Jones this morning and we're both focused on fighting for a fair deal for Scotland and Wales.
"You only get that with Labour – the Tories want to strip back investment and the SNP wants to break up the UK, which would mean the end of the Barnett formula."