Alistair Darling admitted tonight that Labour's planned cuts in public spending will be "deeper and tougher" than Margaret Thatcher's in the 1980s, as the country's leading experts on tax and spending warned that Britain faces "two parliaments of pain" to repair the black hole in the state's finances.

The Institute for Fiscal Studies said hefty tax rises and Whitehall spending cuts of 25% were in prospect during the six-year squeeze lasting until 2017 that would follow the chancellor's "treading water" budget yesterday.

Asked by the BBC tonight how his plans compared with Thatcher's attempts to slim the size of the state, Darling replied: "They will be deeper and tougher – where we make the precise comparison I think is secondary to an acknowledgement that these reductions will be tough."

The shadow chancellor, George Osborne, seized on the first admission by the chancellor that Labour was planning greater austerity than that achieved by Thatcher's chancellors Geoffrey Howe and Nigel Lawson. "Labour has been found out. Gordon Brown is basing his election campaign on the claim that Labour can go on spending. That is completely blown apart by Alistair Darling's admission, under pressure, that Labour's own budget numbers imply deep cuts. But why didn't he admit that yesterday? Twenty-four hours on, this empty budget has completely unravelled and Labour's failure to act will hit families hard."

Robert Chote, the IFS's director, said he was wary of the chancellor's claims that he could raise £11bn through efficiency savings, and added that capital investment in Britain's infrastructure would bear the brunt of the cuts. Current Treasury plans implied reductions in capital spending of almost 15% a year for the next four years, Chote said.

The IFS used its post-budget analysis to spell out what was in store for Whitehall departments, but said there appeared to be only a modest difference between the plans of the two main parties.

Assuming that the Conservatives wanted to eliminate Britain's structural deficit over a five-year parliament, a David Cameron government would have to find an extra £8bn of savings.

The thinktank said Labour's plans implied a cumulative decline of 11.9% in departmental spending on public services and administration over four years, a cut of £46bn in inflation-adjusted terms.

But a two-year government pledge to protect spending on the NHS and schools, and to raise overseas aid to the UN target of 0.7% of national output, will result in deeper cuts of 20% for those departments not protected, the IFS said. If the government continued to spare health and education for a further two years, departments such as transport, defence and the Home Office would face budget reductions of 25%.

The IFS said that the planned austerity would reduce public spending as a share of the economy from just over 27% to below 21% and return it to its level in the late 1990s, when it began a decade-long rise. A government that wanted to slash the deficit without inflicting such deep cuts would have to raise taxes or reduce welfare payments instead, the IFS added.

Chote said there was a lack of clarity about how either Labour or the Conservatives planned to tackle deficit reduction. "There are an awful lot of judgments still be made, or revealed, notably with regards to public spending over the next parliament. This greater-than-necessary vagueness allows the opposition to be vaguer than necessary, too."

The budget, Chote added, had failed to provide a detailed picture to voters and the financial markets of the "fiscal repair job" in prospect after the election.

"Of the £46bn a year of real cuts in public services spending that we think budget forecasts would require by 2014-15, the government would presumably claim to have 'found' about £20bn by 2012-13 from pay restraint, cutting programmes and efficiency savings. We should be wary of some of these claims, particularly on efficiency.

"First, because it is not obvious that the efficiency savings would be delivered. Second, and more fundamentally, if they are cutting out genuine waste we would expect the government to try to achieve most of these efficiencies even if it was not having to cut public spending overall."

The IFS said that the slight improvement in the state of the public finances in recent months had reduced the size of the structural budget deficit – borrowing that will not be eliminated by faster growth – from £73bn before the budget to £67bn.

Chote said: "Presented with this good news, the chancellor had a choice: to give away his good luck in pre-election bribes, or to bank it and bring government borrowing down more quickly as the recovery takes hold. Perhaps in the face of pressure to do otherwise by the "forces of hell", he has sensibly chosen to do the latter."