Who ever is elected on 7 May, his or her goverment will fail to balance the books before the end of the decade, according to the International Monetary Fund (IMF)
The organisation said "uncertainties" surrounding next month's general election implied that austerity would be less severe than currently forecast.
In a blow to Tory plans to achieve an overall budget surplus by 2018-19, IMF forecasts showed the UK would still be running a deficit of around 0.3% of gross domestic product (GDP) by 2020 - or around £7bn under implied spending plans - instead of a surplus of the same amount.
Britain's deficit stood at more than £90bn last year, or 5% of GDP.
The IMF said spending on public services was likely to be higher over the next five years than suggested by the Government in the Budget.
"On the expenditure side, given uncertainties pertaining to the May elections, a slightly slower pace of consolidation than that in the Budget is assumed for fiscal year 2016-17 and beyond," the IMF said in its twice-yearly Fiscal Monitor.
The Office for Budget Responsbility (OBR), the government's fiscal watchdog, said last month that Chancellor George Osborne's plans to end Britain's spending squeeze a year early would put the UK on a "rollercoaster profile" for public services spending, with a massive squeeze followed by a £20bn splurge at the end of the decade.