Britain's leading economics thinktank yesterday accused all three main parties ‑ and particularly Labour ‑ for failing to come clean over the scale of tax rises, welfare cuts and spending retrenchment necessary after the election.

In an attack on the "vague" plans sketched out by Labour, Conservatives and Liberal Democrats, the Institute for Fiscal Studies also claimed the Tories were planning the sharpest spending cuts since the second world war, while the Labour and Lib Dem spending slowdowns amounted to the biggest retrenchment since the IMF crisis in the mid-1970s.

The IFS said that no party had gone "anywhere near identifying" the cuts they will need to meet their various deficit reduction timetables.

The attack leaves all three parties with awkward questions to answer ahead of tomorrow's televised leaders debate, focused on the economy. In a damning assessment the IFS says: "Repairing the public finances will be the defining domestic policy task of the next government. For the voters to be able to make an informed choice in this election, the parties need to explain clearly how they would go about achieving it. Unfortunately they have not. The opposition parties have not set out their fiscal targets clearly, and all three are particularly vague on their plans for public spending. The blame for that lies primarily with government for refusing to hold a spending review before the election."

Concern over Britain's deficit intensified last night as markets fell heavily when Greece was downgraded to "junk" status by credit rating agencies. Shares in London saw their biggest drop of the year, with the FTSE 100 closing down 150 points, amid fears that the Greek crisis was spreading to other heavily-indebted countries. Standard & Poor's cut Greece's credit rating to junk status and also downgraded its view of Portugal.

With Labour using its election broadcast to claim the Tories were planning secret cuts, the business secretary, Lord Mandelson, came under pressure at the party's morning press conference to set out Labour's own cuts programme. The IFS report suggests that by 2014-15, the Tories will need to find £64bn a year cuts in their unprotected areas, but the Conservatives have only detailed less than a fifth of the cuts that they would need. Labour needs to find cuts of £51bn, but has identified only an eighth. The Liberal Democrats need to identify almost £47bn cuts, and have identified a quarter.

The different cuts required by each party reflects the degree to which each party plans to make tax rises, as opposed to spending cuts, take the strain of reducing the deficit, as well as the speed with which they want to cut the deficit.

But the IFS expressed scepticism about the prospects of the next government ‑ of any political colour ‑ being able to deliver spending cuts on a scale not seen for at least 30 years through proposed efficiency savings.

"This may suggest that all the parties are being overambitious in the extent to which they expect spending on public services to take the strain," said Robert Chote, the IFS director. "If so, the next government may rely more on further tax increases and welfare cuts than any of the parties are willing to admit to beforehand."

The IFS also estimated that Labour's package helped the poorest most, and the Tories was the least progressive. The report goes out of its way to criticise David Cameron's plans not to go ahead with Labour's planned rise in national insurance. It estimates that Labour is likely to find an extra £7bn in tax per year by 2016-17 to meet its goals, the Conservatives will need to raise taxes by £3bn, and the Liberal Democrats have probably set out the tax rises they need. The IFS said Labour plan to cut the deficit with a 2 to 1 ratio between spending cuts and tax increases, the Liberal Democrats 2½ to 1 and the Conservatives 4 to 1.

While the IFS would not be drawn on which taxes might be raised after polling day, many City analysts believe the likeliest target would be an increase in VAT from its current rate of 17.5% to 19% or 20%. The Treasury has already drawn up contingency plans for a VAT increase in the event that an incoming government should deem it necessary to bring down the £164bn deficit amassed last year.

Labour says the spending review covering the departmental totals for the three years 2011-12 to 2013-14 was delayed due to economic uncertainty, and claims that it has set out a clear mix of efficiency savings, cuts and tax rises to halve the deficit by then.

The shadow chief secretary, Phillip Hammond, conceded that none of the parties had given enough detail: "I don't think any party has identified in detail how they will reduce public spending over the course of the coming parliament."

He blamed the government for failing to hold a spending review before the election. "If the government published a comprehensive spending review they would set a baseline and that would be a challenge to the opposition parties then to respond to that."

But the chancellor, Alistair Darling, said: "When I said the recession would be the most serious for 60 years, I was criticised by the Tories. When I said if we took action, we get growth again by the end of the year, I was attacked by the Tories. But when you look at what happened, events have proved my judgments right."