Alistair Darling has said Scotland would be taking an "amazing" risk by voting for independence and insisted "the uncertainties are not worth gambling on".
The former Labour chancellor of the exchequer warned of the "immense" economic difficulties the country would face as a separate nation.
Mr Darling is seen in some quarters as a front-runner to lead the ''no" campaign in a referendum on independence, but in an interview with The Observer he said he was not interested in the role.
"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.
The MP for Edinburgh South West said a newly independent Scotland would face deep economic uncertainty against a backdrop of continued global turmoil.
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.
On the other hand, joining the euro would mean Edinburgh would have its own interest rates set by the European Central Bank, while setting up a new Scottish currency would be a huge gamble.
"If you have a single currency area you come back to having an economic if not a political union," Mr Darling 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."
"The downsides are immense, the risks are amazing, the uncertainties I just don't think are worth gambling on. There are times when you should gamble and there are times when you shouldn't."
His comments were dismissed by SNP treasury spokesman Stewart Hosie, who accused Mr Darling of spreading "Unionist scare stories".
“The anti-independence parties really are struggling if they see Alistair Darling, the former Labour Chancellor in a government which crashed the economy, as a credible champion for their campaign," Mr Hosie said.
“Thankfully, people in Scotland are increasingly seeing through the Unionist scare stories and the attempts to talk down their own ability to make a success of themselves, their communities and their country.”