An independent Scotland would face a "fiscal squeeze" due to its ageing population, lack the firepower to bail out its banks, and struggle to finance low interest rates, Treasury Chief Secretary Danny Alexander has warned.
Speaking on the panel of a Better Together event at the Scottish Liberal Democrat conference, Mr Alexander attacked the Yes Scotland campaign.
He said: "You can't turn on a television at the moment without seeing Alex Salmond's face looming out at you, making his argument.
"But while he is not the invisible man, he has an invisible plan.
"He does not explain what the basis of his arguments are."
Mr Alexander said there were serious questions over a number of areas related to Scotland's financial future, should the country vote for independence in 2014.
These included the notion of a currency union within the UK, which he described as an attempt to "recreate the initial model of the Eurozone on our islands" and Scotland's ability to bail out its banks.
"I don't think that Scotland on its own would have the fiscal firepower to take those sort of decisions for itself," he said.
Turning to spending and borrowing, he said the ageing population would result in "a big fiscal squeeze on an independent Scotland."
Mr Alexander went on: "Without the stability of the UK...there would be serious questions about whether an independent Scotland could finance the low interest rates which the UK - with its deeper pockets and wider fiscal power - is able to command."