The Institute for Fiscal Studies (IFS) has said none of the three largest parties at Westminster has come "anywhere close" to making clear where cuts would be made to meet their deficit reduction targets over the next four years.
This, it said, was despite the parties' plans implying the deepest cuts in spending since the 1970s and - in the case of the Conservatives - the biggest one-year reduction in public spending since demobilisation at the end of World War II.
Vince Cable outlines Lib Dem plans to deal with public sector benefits
Miles Templeman, director general of the Institute of Directors [IoD], accused politicians of "complacency", telling BBC Radio 4's Today programme they had failed to describe "the scale of the deficit reduction we're going to need".
Long-term plans on "infrastructure investment, rebuilding our skills position and deregulating" were also not being explained.
In a keynote speech on the economy in Edinburgh, Chancellor Mr Darling said the government had been "very clear" about the need to cut Britain's record deficit.
"There can't be that many chancellors who have gone into an election saying that the spending conditions are such that we'll have one of the toughest settlements in many, many years," he said.
Repeating Labour's commitment to halve annual borrowing within four years, while protecting spending on the NHS, schools and police numbers, he added: "The rest of spending will face a much tougher regime."
Mr Darling said no spending review was carried out in the past year because areas like unemployment and debt interest were expected to be higher than they have turned out.
He added that shares in the part-nationalised banks were showing a £5bn profit and could help pay off the national debt.
Mr Darling later told BBC Radio 4 the IFS had referred to projections up to 2017 - in the parliament after next.
"You're asking a lot of any chancellor to set a budget for eight years time."
Shadow chancellor George Osborne told the IoD's annual conference that it was "pathetic" to suggest the government could not save money by freezing IT projects and not filling vacant posts.
"It's ludicrous to claim that wasting money is central to economic recovery," he said.
Mr Osborne repeated pledges to impose a public-sector pay freeze, cut back on the availability of child trust funds and child benefits, and reduce incapacity benefit for those judged fit to work - and said the Tories would go further.
"Have we ever pretended to you that these are all the things we need to do? No. Anyone elected next Thursday will have to do more," he said.
He was setting out plans for a new regulation system, a bank levy and the creation of a new economic crime agency.
Lib Dem Treasury spokesman Vince Cable had earlier told the audience that politicians needed to break down some "basic taboos", such as cuts in welfare benefits.
The Lib Dems had plans to scrap child trust funds, reduce winter fuel payments for the under 65s and address public sector pension funding, he said.
Mr Cable said it made "absolutely no sense" to cut back the budgets of "low-profile departments to protect bureaucracy in high-profile departments", such as education or health.
But he added: "In the short term, you shouldn't be doing anything in government that pushes the economy into a double-dip recession."
He was dismissive of Conservative claims to be able to reverse the government's planned National Insurance rise by making efficiency savings.
"Efficiency has become the new politically correct word for sacking people and cutting services," said Mr Cable.
Speaking earlier at a campaign event in Wakefield, Mr Cameron had said: "The responsibility for the start of this problem and not being frank with the public lies squarely at the door of Gordon Brown."
However, Labour have described the Conservative leader's comparison of the UK economy to Greece, where the credit rating has been downgraded to "junk", as "economic illiteracy".
Business Secretary Lord Mandelson also addressed the IoD conference, saying the election was not about public versus private sectors but about driving demand in the economy.
"Tough decisions are needed and I don't apologise for telling anyone in the public sector that belt-tightening now is the order of the day," he said.
He said he did not accept criticism of Labour's plans to raise National Insurance.
"Don't tell me that taxes should not go up and at the same time tell me that the number one priority is fixing the public finances."
The BBC's political editor Nick Robinson said the parties' reluctance to talk about the details of future cuts was understandable given the closeness of polling day, but it risked another crisis in confidence in the political system if people felt they were only told the truth after the election.
Later, Lib Dem leader Nick Clegg will argue that his party is the only one committed to scrapping student tuition fees, although he will admit the crisis in the public finances means this cannot be done "overnight".
He will say his opponents want to allow universities to raise the current cap on fees, and this would be a "disaster".
"Students can make the difference in countless seats in this election," he will say.
"Use your vote to block those unfair tuition fees."
Meanwhile, the latest opinion polls continue to suggest no single party is on course to win an outright majority.
A Populus poll for the Times puts the Conservatives on 36%, up four points on last week, the Lib Dems down three points on 28% and Labour down one on 27%.
A YouGov daily tracker poll for the Sun puts the Conservatives on 33%, Labour on 29% and the Lib Dems on 28% while a Comres poll for the Independent and ITV News puts the Tories on 33% and Labour and the Lib Dems level-pegging on 29%.
The BBC's head of political research David Cowling said the polls were following the pattern of the past 10 days, with the battle a clear three-horse race and a hung parliament remaining a distinct possibility.
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