The chancellor will need to find an extra £5bn of spending next year to meet the government's recent pledges, the Institute of Fiscal Studies (IFS) has warned.
A new report, published by the IFS ahead of tomorrow's Spending Round, finds an extra £5bn is needed just to avoid cuts to other public services.
It estimates that pledges on schools, police, NHS, defence and overseas aid will require at least £9bn more next year compared to this year.
It warns the Government has little idea how sustainable big spending decisions will be.
Figures published by the Office for Budget Responsibility (OBR) at the Spring Statement gave the Government £15bn of headroom against its target to keep borrowing below 2% of national income next year.
However, the IFS warned that making major fiscal announcements without new OBR forecasts could put public finances at risk.
Ben Zaranko, an author of the report, said: 'Significant sums are already pledged for schools and the health service, more is coming for the police, and there are commitments on defence and aid spending.
This spending round will, though, also need to consider very carefully the clear and growing need for funding in our justice system and in local government, including social care, where cuts over the past decade have been particularly severe.'
