A CURRENCY union between an independent Scotland and the rest of the UK would not last if the two economies diverge, an SNP MP has suggested.
Angus MacNeil said his party's preferred option, a sterling zone, would be for the long-term "as long" as the newly separate countries remained in step.
The pro-Union Better Together campaign last night accused Mr MacNeil of letting the "cat out of the bag".
However, a Scottish Government spokesman said ministers "had not suggested any (time) limit" on a currency union and added there was no suggestion the two economies would diverge.
The SNP argues that a sterling zone would have significant benefits for both an independent Scotland and the remaining UK.
However, analysis by the Treasury released earlier this year argued the markets would reject such an arrangement unless they believed it to be permanent.
The document led all three of the main pro-Union parties at Westminster to rule out such a deal.
At the weekend, however, the No campaign suffered a blow when an unnamed UK Government minister suggested the stance was a bluff, saying "of course there would be a currency union".
During a debate on Scottish independence at Westminster Mr MacNeil, the MP for the Western Isles, was asked how long any currency union would last.
He said: "I would expect it to last my lifetime. I can't say much beyond that. It's long-term, as long as the economies stay as they are - quite convergent, which they are."
A spokesman for Better Together said: "Angus MacNeil has let the cat out of the bag.
"One of the many reasons why a currency union would never happen is because it would be so uncertain. At least Angus MacNeil is honest about that fact.
"The Prime Minister, Chancellor, Shadow Chancellor, Chief Secretary to the Treasury and the Permanent Secretary of the Treasury have all said a currency union would not happen.
"What we need from the Nationalists is their Plan B on what would replace the pound. Would we rush to adopt the euro or set up a separate unproven currency?
"The idea that voters can be expected to go to the polls blind on this most fundamental issue is simply not credible."
Yesterday, Scottish Secretary Alistair Carmichael attempted to play down the significance of the anonymous minister's briefing. He said every political campaign has "difficult days" but "it does not add up to a campaign in crisis".
He also said the pro-Union side had to be "more energetic" in making the case for Scotland remaining part of the UK.
Meanwhile, Finance Secretary John Swinney welcomed comments from economist Professor Anton Muscatelli, calling for a currency union between an independent Scotland and the rest of the UK.
Professor Muscatelli reiterated his view that "a successful currency union would actually be in the interests of both sides - and especially the rest of the UK."
Mr Swinney said: "This is a significant and welcome intervention from Professor Muscatelli, who has carefully considered the facts and concluded a currency union would be interests of both Scotland and the rest of the UK.
"This is also a further blow to the credibility of the No campaign's position, which has completely fallen apart over the last few days."
Meanwhile, Mr MacNeil has also said removing Scottish MPs from Parliament on the day Scotland becomes independent would save taxpayers £50 million.
He insisted that if a Yes vote was returned in the referendum, then Scottish MPs should vacate their seats and constituencies north of the border be abolished on the date Scotland breaks away.
Mr MacNeil was introducing his Scotland (Independence) (Westminster Representation) Bill, which was unopposed.
It is unlikely to make further progress without Government support and due to lack of parliamentary time.
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