Sunak said it was clear the prime minister had “opened the door to raising employer national insurance contributions, including on pensions, and fiddling the figures so he can borrow more”.
The budget this month is the first major financial event since Labour won a landslide victory in the general election in the summer.
It will set the direction of the Labour government, which has had a difficult few months following the summer riots, a row over donations and a reshuffle of the prime minister’s Downing Street team.
Chancellor Rachel Reeves discussed the budget at a meeting of cabinet ministers on Tuesday.
Since taking office, Reeves has warned of “difficult decisions” on tax, spending and benefits, to plug a £22bn “black hole” she claims was left by the Conservatives.
Labour ruled out raising income tax, national insurance and VAT in its election manifesto, and Sir Keir said he would stick to those commitments at Prime Minister’s Questions on Wednesday.
But Sir Keir did not say whether his pre-election promise applied to national insurance paid by employers.
