Alistair Darling will demand "a very big apology" from David Cameron for deliberately misleading the electorate about the state of the nation's finances if, as Darling expects, government borrowing figures released on Monday are better than forecast.
In an interview with the Guardian, the former chancellor accuses Cameron of deliberately exaggerating the scale of the problems so as to press ahead with pre-arranged plans to raise taxes and cut the size of the state.
Cameron's new Office of Budget Responsibility is due to publish a fresh borrowing forecast independent of the Treasury on Monday, and Darling suspects it will be lower than his own forecast in March.
A cautious politician during his three years as chancellor, Darling is personally affronted that he has been accused by the Tories of massaging forecasts and ignoring civil service advice. He is involved in a battle to defend his and Labour's legacy as responsible economic stewards and is taking a political gamble by raising the issue ahead of Monday.
He said: "If, when we see the borrowing figures on Monday, they turn out to be even better than I forecast in the budget, Cameron will have been found out to have misled people, and that is an extremely serious charge to lay against a prime minister. We will be due a very big apology and I'm going to get it from him."
He points out that the main two figures published since March have shown borrowing £10bn lower than forecast and growth marginally higher.
Darling said: "If it turns out that borrowing is lower than I thought then that puts a completely different complexion on things because what he has been doing is exaggerating the scale of the difficulties we face in order to justify the things he has wanted to do anyway."
In the interview Darling also discloses that he wanted to abolish identity cards in the pre-budget report last autumn, as part of a programme of spending cuts designed to show Labour's fiscal responsibility.
He also says he will take his share of the blame for the political class failing to tell the electorate about the scale of the cuts after the election. "It would be nice to live in a world in which you could have a candid debate about these things, but it's difficult. I will take my share of the blame.
"I think we could have got through this recession and won the election. There were bits of medicine we could have administered last year that would have made things easier. Had we gone further in saying to people round the cabinet table we are not going to do this [project], and we are going to make a virtue of not doing it, it would have been easier for us."
He also hints at irritation with the way the governor of the Bank of England, Mervyn King, intervened, saying: "I do think the Bank has to be terribly careful it does not come into party politics because that could be very, very damaging to it."
He also admits he is concerned by lack of growth in Europe, adding: "I am afraid that fiscal conservatism is now dominant." Revealing his fears for the euro's future, he says: "The real problem in the euro area is that because they do not have an economic and political union to back their currency, they just don't recognise that if Germany continues not to do something to stimulate the poorer economies, it is going to undermine the whole euro."
He warns if the Conservatives and Liberal Democrats take "large chunks out of our economy" while Europe is growing slowly, "the inevitable result is that we will bump along the bottom for years, and at worst you could derail the economy, but whatever happens jobs are going to go."
He also warns Cameron against scaremongering, saying he may depress investor and consumer confidence: "The blacker the picture he paints, the more likely it is to become self-fulfilling."
He discloses he was opposed to trying to form a coalition with the Lib Dems and that he held coalition talks with Vince Cable for a mere 20 minutes. "We agreed we had probably met each other for long enough because they had already done a deal with the Tories," he said. He is scathing at the way in which Cable's previous commitment to oppose cuts this year was jettisoned. Darling said: "The sniff of power was so great that £6bn was not going to get in their way."
He also admits his party had no chance of winning the election. "Labour had lost the election before the starting gun was fired. Elections are won over the course of a parliament and there were just too many things we were getting wrong. We were giving the impression that we were not in complete control. The biggest single group of the electorate that felt left out were people on pretty moderate incomes with no children, especially living on the edges of more prosperous cities."
Asked if he accepted the criticisms that he was insufficiently party political as chancellor, he said: "No, I do not. Your actions should speak for themselves."
In an apparent dig at Labour leadership contenders Ed Miliband and Ed Balls, he says "I've heard a lot of people on my side say we should be tougher on the banks. OK, that's fine. Tell me what, and also tell me why you never mentioned it in the last three years."