Former chancellor Alistair Darling has warned of the "immense" economic difficulties Scotland would face if it voted for independence.
The Scottish MP for Edinburgh South West said the risks would be "amazing" and not worth gambling on.
Mr Darling has emerged as a leading candidate to head the pro-Union campaign but, in an interview with The Observer, said he was not interested in taking on such a role.
However, he delivered a powerful rebuttal of the arguments advanced by the Scottish National Party administration at Holyrood for independence.
"The downsides are immense, the risks are amazing, the uncertainties I just don't think are worth gambling on," he said.
"There are times when you should gamble and there are times when you shouldn't."
Mr Darling, who was chancellor under Gordon Brown during the 2008 banking meltdown, said a newly-independent Scotland would face deep economic uncertainty against a backdrop of continued global turmoil.
He said that if Scotland retained sterling it would remain tied to England and Wales and face similar challenges to the eurozone now, which lacks formal political union.
"If you have a single currency area you come back to having an economic if not a political union," he said.
"So you go through all the trauma and expense of leaving the Union, only to come back and discover that because you want to be part of this common currency you are back to where you were. I just don't see the sense of that."
Leaving sterling and joining the euro would not give Edinburgh independence, he said, as it would still not set its own interest rates and would be more beholden to Brussels.
And he said that for Scotland to set up its own currency would be a huge gamble.
"You would be a brave country indeed to say, 'here is our new currency, we are not actually sure how much it is going to be worth after the first day's trading'," he said.
"It would be one hell of a risk."
Mr Darling urged the Treasury to publish its assessments of the economic impact of the break-up of the Union. But he said he would not lead the No vote in the planned referendum.
"I will play my part, certainly, but I don't want to do that. I am too busy as a Westminster MP. This campaign has to be run in Scotland," he said.
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