Bank of England governor Mark Carney has told trade unions that currency union in the event of Scottish independence would be “incompatible with sovereignty”.
Mr Carney told the TUC conference that a currency required a centralised bank and shared banking regulations.
Common taxation and spending were also needed, he said.
The SNP said currency union was “in the best interests of both an independent Scotland and the rest of the UK”.
It added that currency union plans had been considered in detail.
For their part, pro-union campaigners said a shared currency would be “bad for Scotland”.
The Scottish National Party (SNP), which wants to keep the pound in the event of independence, said that its plans had been “considered in detail” by the Fiscal Commission, a working group of the Scottish government.
An SNP spokesperson for Scottish finance minister John Swinney said: “Successful independent countries such as France, Germany, Finland and Austria all share a currency – and they are in charge of 100% of their tax revenues, as an independent Scotland would be. At present under devolution, Scotland controls only 7% of our revenues.”
The Conservatives, Labour, and the Liberal Democrats have all come out against a currency union with an independent Scotland.
The SNP spokesperson said that “the political position of the three Westminster parties… will of course change after a Yes vote.”
“And as the momentum builds behind the Yes campaign, their currency bluff has well and truly been called,” the spokesperson added.