Danny Alexander has accused the Scottish Government of basing its plans to keep the pound after independence on "pure fantasy".

The Liberal Democrat Chief Secretary to the Treasury said the SNP was running an "assertion factory" that was "crumbling under the glare of the real world".

His comments come a day after The Herald revealed a former Bank of England analyst warned the price of retaining sterling could be the surrender of almost all fiscal sovereignty.

"An independent Scotland almost certainly means a new currency," the report by Mark Allan, a specialist with Axa who at one time gave advice to the Bank's Monetary Policy Committee, also predicted.

The Scottish Government says it wants to keep both the pound and the Bank of England as its so-called lender of last resort.

But earlier this week the UK's central bank denied it was having any dialogue with Scottish ministers over a sterling zone.

Mr Allen warned the cost of any currency union would include strict conditions, including possibly a common tax policy.

He said if an independent Scotland was to surrender almost all fiscal sovereignty to keep sterling it would face questions over the point of independence.

He added "Facing such a choice, an independent Scotland would surely have to issue its own currency and make its own way in the global financial markets."

Mr Alexander told The Herald: "The SNP's assertion factory is crumbling under the glare of real world scrutiny.

"Just like its shambolic position on the EU, the Scottish Government's assertion that an independent Scotland will keep the pound on what-ever terms it chooses is pure fantasy.

"I have been clear for over a year now that the lesson of the eurozone crisis is that any shared currency must have strong, centralised controls over tax, borrowing and spending."

Former chancellor Alastair Darling pointed to this week's admission by Scottish ministers that they would have to negotiate with the EU over issues, including the euro.

"How could you join a currency union when you don't know whether or not you've got to join the euro?" he said.

"The deceit here is that for years the nationalists have said 'you don't need to worry about all of this', now the same people are saying 'well, actually we'll get in overnight, it's a mere nod and wink and no problem at all'.

"I don't believe a word these people are saying because I don't believe they've actually done any preparation in relation to this. They're flying by the seat of their pants."

Scottish Conservative leader Ruth Davidson accused Scottish ministers of having "no idea what people in Scotland would have in their pockets under separation".

Labour's Shadow Finance Secretary Ken Macintosh said: "This report reveals why the First Minister suddenly finds it so hard to talk about his previous plans for a fiscal stability pact.

"When serious voices are raising concerns about the viability of their plans the SNP have to take notice and come up with a credible response."

Last night, the Scottish Government refused to be drawn on Mr Alexander's comments.