An independent Scotland would need to ensure a formal currency union arrangement to keep the pound was completely viable, like being pregnant, Bank of England governor Mark Carney has warned.
The central bank chief made the unusual comparison speaking before the Treasury select committee about the implications of Scottish independence and potential risks if voters choose to break away from the UK in September's referendum.
"Viability in a currency union is a bit like being pregnant in the sense that you can't be half viable in a currency union. You need all the components of a currency union. You have to look at the whole package," he told MPs.
Carney argued that there would be "advantages" to an independent Scotland if it became part of a EU banking union but it would be "half-viable" as it would not have a common deposit guarantee scheme and lender of last resort negotiations would have to be made.
This comes after Carney warned in a speech made in Scotland that a "successful currency union requires some ceding of national sovereignty", while the British government argued that an independent Scotland would be handing over control of its interest rates and borrowing levels to a foreign nation.
However Carney warned that the Scottish economy is "more volatile" and that there was a "distinct possibility" that banks like the Royal Bank of Scotland would be forced to move their headquarters out of an independent Scotland.
Carney suggested the rest of the UK could end up bailing out Scotland if it was hit by a financial crisis.
He told MPs: "There's famously a no-bailout clause in the Maastricht Treaty but when push came to shove with risks in the periphery that was not fully credible because the spillover effects of not bailing out, or not assisting in some sort of arrangement, were judged, in the teeth of a global crisis, to be too great to risk and so there were extraordinary efforts made, and appropriate efforts, made by European officials to avoid that.
"The issue for the rest of the UK would be would it be credible, I'm not predicting this but one has to plan for contingencies, would it be credible to stand by if an independent Scotland were to be in fiscal difficulties?
"If it is not credible then the structure of the currency union obviously is less than perfect but it is also taking on a contingent liability or responsibility to the rest of the UK balance sheet. So, these are all issues that have to be thought through in advance."
The Bank of England governor confirmed to SNP committee member Stewart Hosie that Scotland and the rest of the UK's economies are "highly correlated", which is "beneficial" for a currency union arrangement.
Carney dismissed suggestions that he thought a currency union was a bad idea.
"I have been absolutely clear and I will continue to be clear that what I have tried to do, what the Bank has tried to do, and what we will continue to do is to draw attention to economic issues which the interested parties can address and then others can make the judgments," he said.
"At no time have I said, or will I say, that I do not support or that I advocate a currency union, either side of the debate. When it was suggested that he had created a "less than optimistic picture", he told MPs his intention was to draw attention to the underlying economic issues.
A fierce argument has erupted over the prospects of an independent Scotland keeping the pound in a currency union arrangement with the rest of the UK after three main political parties said that it would not be possible.
Chancellor George Osborne ruled out any chance of an independent Scotland joining a formal currency union, in a move which was backed by Labour and the Lib Dems.
Scotland's Deputy First Minister Nicola Sturgeon dismissed the warnings as the "Westminster establishment" trying to lay down the law to Scotland and warned the move would backfire on the pro-Union parties.
Osborne said: "The pound is one of the oldest and most successful currencies in the world. I want Scotland to keep the pound and the economic security that it brings,"
"The UK works, in good times and also in bad. Together we have faced the worst economic and financial crisis since the Great Depression.
"But we avoided the economic collapse other nations around us in Europe faced because together, we had the strength to confront our problems and overcome them."
Carney told the Treasury select committee earlier today that the Bank of England could not directly stop wealthy foreign buyers pushing up house prices by snapping up expensive properties in the capital.