Speaking at a business in the north-west of England later, Sir Keir, 62, will accuse the Reform leader of pledging unfunded tax cuts which, he will warn, could spark an economic meltdown.
“In opposition we said Liz Truss would crash the economy and leave you to pick up the bill,” he is expected to say. “We were right. And we were elected to fix that mess.
“Now in government, we are once again fighting the same fantasy – this time from Nigel Farage.
“Farage is making the exact same bet Liz Truss did. That you can spend tens of billions on tax cuts without a proper way of paying for it. And just like Truss, he is using your family finances, your mortgage, your bills as a gambling chip on his mad experiment. The result will be the same.”
Thursday’s speech is further evidence that right now the prime minister sees Farage as his principal political adversary.
On Tuesday, Farage pledged more generous benefits for pensioners and parents – but it was a commitment to raising the threshold at which someone starts to pay income tax from £12,570 to £20,000 that had some economists questioning whether his sums added up.
The Institute for Fiscal Studies said the policy could cost between £50bn and £80bn a year – and that Reform had not spelled out how they would raise the cash.
“Of course they don’t have to do that yet – we’re not at a general election,” said IFS economist Stuart Adam. “But at some point, if they’re going to be a party of government, they would have to make those numbers add up.”
