Alex Salmond claims Danny Alexander has been "caught red-handed" with "scaremongering" claims about the impact of Scottish independence.
The SNP leader told the BBC the Treasury's calculations had been "blown to smithereens" by the same academic whose work underpins the British Government's analysis.
Professor Patrick Dunleavy of the London School of Economics said Treasury figures about the cost of setting up a new independent government were "bizarrely inaccurate".
Mr Salmond seized on Professor Dunleavy's words, saying: "We have the Chief Secretary to the Treasury left with no credibility whatsoever. He’s been caught red-handed engaged in a scaremongering campaign.”
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Five people were killed and dozens more were injured after a car and knife attack in the capital on Wednesday afternoon.
Among those expected to attend the ceremony will be former US President Bill Clinton and ex-Northern Ireland first minister Peter Robinson.
Five people have been killed and 40 injured after a car and knife attack in London on Wednesday.