Alistair Darling has claimed Alex Salmond was a "complete fool" if he thought the Royal Bank of Scotland's takeover of ABN Amro was good for Scotland.
The former Labour Chancellor, speaking before an appearance at the Aye Write! literary festival in Glasgow, said the First Minister was an exceptional politician but should not be hyped as "infallible".
He said: "This is the man who wrote to RBS saying he had looked at the deal with ABN Amro and was confident it was good for Scotland. Well, if he honestly thought it was good for Scotland, he's a complete fool because it brought the entire RBS edifice crashing down. So, he does get judgment calls wrong."
In an exclusive interview with The Herald, Mr Darling pledged to play a full part in the campaign to keep Scotland in the United Kingdom.
The pro-Union parties have been keen to get the former Chancellor at the forefront of the campaign and he said he would be involved because it is something about which he "passionately" cares, and that he was optimistic the Unionists would win the day because "we have the right arguments".
He said he "simply cannot abandon the House of Commons for two-and-a-half years", but would spend more time in Scotland as the 2014 date set by Mr Salmond comes nearer.
Mr Darling warned that the financial risks of separating from the UK at a time of global uncertainty were "immense".
He said: "Scotland has much to contribute, but when the centre of economic gravity is moving remorselessly from the West to the East to the South, the big question is how we best equip ourselves to build and develop new industries to meet that challenge.
"When we come to the referendum, it's not like electing a Government which, if it turns out to be no good, you then put in another lot. Separation means once you go, you go. You can't come back."
Mr Darling said Labour's defeat at last May's Holyrood election "had been a long time coming".
"We just assumed people would come out and vote for us. It never occurred to us they might go somewhere else because we didn't have a particularly compelling or attractive vision of what we were about."
He denied Labour's strongest candidates stood for Westminster rather than the Scottish Parliament.
He said there were "good people in Holyrood, and there are people in Westminster who are perhaps not the best advert for what you're doing".
A spokesman for Mr Salmond said: "This is a laughable attempt to rewrite history by Alistair Darling.
"He was the Chancellor responsible for banking regulation and its failure at the critical time, and he was the Chancellor responsible for signing off the ABN Amro deal.
"Labour gave Fred Goodwin his knighthood, and Mr Darling's contacts with Fred Goodwin were far more extensive than the First Minister's.
"Fred Goodwin was an adviser to Alistair Darling as Chancellor, and was still a member of a key Treasury body advising Labour months after the banking crisis and quitting RBS."