INDEPENDENCE would threaten a major economic crisis for Scotland and be regarded in history as being as great a mistake as the decisions that led to the Great Depression of the 1930s, according to banking giant Deutsche Bank.

In the most scathing anti-independence report to date by a major financial institution, analysts describe as "incomprehensible" why anyone would "want to exit a successful economic and political union with a G5 country - a union which another part of Europe so desperately seeks to emulate - to go it alone for the benefit of what exactly?"

Last night a spokesman for John Swinney, the Scottish Finance Secretary, swiftly hit back, saying: "With a Yes vote on Thursday, Scotland will become independent from stronger economic foundations than any other nation in history."

He stressed Scotland was one of the world's wealthiest nations with wealth per head higher than France, Japan and the rest of the UK and would start life as an independent country from a strong fiscal position.

"This report would appear not to take account of any of that at a time when more and more people in Scotland are waking up to the fact that we can, we should and we must vote Yes for a more prosperous country and a fairer society," he added.

The bank's intervention came as a second poll in 24 hours gave the No side only a slight lead over the pro-independence campaign. A Guardian/ICM poll put support for Scotland staying in the UK at 51 per cent, ahead of 49 per cent backing for independence when undecided voters are excluded.

Reacting to the latest snapshot, Deputy First Minister Nicola Sturgeon said the battle for Scotland's future was on a knife-edge but "moving in the direction of Yes". "The only poll that really matters is the one next week. We're now within a few days of finding out what people really think through real votes and ballot boxes," she said.

Today, tens of thousands of Orange Order marchers are due in Edinburgh to show their support for the Union; the death of the former Democratic Unionist leader Ian Paisley is likely to make the event more poignant.

In another development, a row broke out over a warning from Jim Sillars that big business in Scotland would face "a day of reckoning" after major firms said that, if there were a Yes vote, several would relocate their registered offices to London and put their prices up for Scottish consumers.

With passions running high as referendum day fast approaches and suggestions are floated this weekend of more retail and telecom giants about to warn of the "inevitability" of increased bills for Scots under independence, Mr Sillars, the former deputy leader of the SNP, delivered a blunt message: "This referendum is about power and when we get a Yes majority we will use that power for a day of reckoning with BP and the banks.

"The heads of these companies are rich men, in cahoots with a rich English Tory Prime Minister, to keep Scotland's poor poorer through lies and distortions. The power they have now to subvert our democracy will come to an end with a Yes."

But Mr Sillars was given short shrift by one company boss, Malcolm Walker, who runs the Iceland supermarket chain, and who dismissed the Nationalist politician's outburst as "bollocks".

"Is that blackmail or a threat or what?" he asked. "That is a stupid thing to say. What's he going to do, close us down?" The company has 71 stores in Scotland and employs 2000 people.

Ian Davidson, a Labour backbencher, claimed the Yes campaign's mask had now slipped to reveal the "ugly face" of nationalism with Mr Sillars telling businesses: vote Yes or else.

"His words are a clear threat to anyone who points out the true costs of separation to the people of Scotland," said the MP for Glasgow South West.

As retail bosses were also said to be planning to publish a letter warning of price hikes should Scots vote Yes, Alex Salmond described recent interventions by business figures as "blatant intimidation from Westminster".

But a spokesman for David Cameron said the Prime Minister simply wanted "stakeholders to set out their views".

It emerged at a private meeting with bosses earlier this week that Mr Cameron issued what one described as a "call to arms", evoking the defeat of Hitler as he addressed more than 100 business leaders.

In the Deutsche Bank report, David Folkerts-Landau, the bank's chief economist, said: "A Yes vote for Scottish independence on Thursday would go down in history as a political and economic mistake as large as Winston Churchill's decision in 1925 to return the pound to the Gold Standard or the failure of the Federal Reserve to provide sufficient liquidity to the US banking system, which we now know brought on the Great Depression in the US.

"These decisions, well-intentioned as they were, contributed to years of depression and suffering and could have been avoided had alternative decisions been taken."

Jeremy Peat, visiting professor at the University of Strathclyde International Public Policy Institute, stressed how any economic benefits from independence would materialise in the longer term and that "in the interim, under a currency union or sterlingisation, we are going to have higher borrowing costs and will have to run a tight fiscal ship to establish credibility; that will take time and effort".

Gordon Brown, the former Labour Prime Minister, said: "Never have so many warnings been given about the damage separatism would cause."

He added: "The world is warning us that a Yes vote will see Scotland fall through an economic trapdoor; a trapdoor from which we could never escape."