The Institute for Fiscal Studies says the Chancellor, John Healey, must find £36bn in tax rises or welfare cuts to offset the hit from inflation to public services and to meet a defence spending pledge, Reuters reported on 12 October.
The figure comes from the same IFS work that warned his borrowing plans would not be realistic if they relied on implicit spending cuts. The IFS said a 'steady-as-she-goes' Budget is not an option.
Helen Miller, the IFS director, said: "It would be a choice to tolerate chunky cuts to the quantity or quality of some public services".
If revenue is raised, the IFS work suggests headroom against the fiscal rules in 2029/30 would fall only modestly, to around £20bn from £24bn in March. Reuters says that is higher than analysts' estimates of about £12bn.
Mr Healey and the Prime Minister, Andy Burnham, say they will keep the previous government's promise not to raise the main rates of tax. Reuters notes that Reeves' previous two Budgets contained more than £60bn of tax rises.