Ed Miliband will urge those who gave the government the benefit of the doubt on the economy to "look again at the evidence", in a speech on Thursday ahead of the Autumn statement.
The Labour leader is expected to say that George Osborne's mini-budget will "mark a crucial moment in the economic course of our country"
“It will be the moment that we learn that the biggest economic gamble in a generation has catastrophically failed."
But he will acknowledge that his own party are not yet economically credible: "So, even as I ask people to look again at the evidence, I know there are questions we still need to answer."
Miliband will argue that the OBR, who are expecting to confirm a rise in government borrowing next year, will deliver a "catastrophic blow" to the credibility of the coalition's deficit reduction strategy.
"After the global financial crisis, Britain was faced with a deficit of over £150 billion. People rightly felt that it had to be dealt with.
"And they are still right to feel so.
"But what has changed, why the Government no longer deserves the benefit of the doubt, is that even on this measure – the central test they set themselves – the plan isn’t working."