The Institute for Fiscal Studies warns that the Chancellor, John Healey, risks setting out borrowing plans that will not hold up if he relies on spending cuts he has no intention of making.
The warning comes in the IFS Green Budget, published on 12 October as a preview of Mr Healey's first Budget on Wednesday 28 October. The report looks ahead to a Budget set against higher inflation and pressure on public spending.
Helen Miller, the IFS director, said that if the Chancellor "simply pencils in implicit cuts that he knows he would never be willing to deliver", his plans to cut borrowing and meet the fiscal rules "won't be realistic".
The IFS also says the government's spending plans rely on ever more ambitious assumptions about productivity.
In a separate piece, titled 'Chancellor Healey meets the bond market', the institute says recent rises in government borrowing costs reflect global factors.