Mr Alexander made clear that the referendum vote would be "respected, full stop, end of story". He did not elaborate on what would happen if there were no agreement between London and Edinburgh by the First Minister's declared date for independence of March 24, 2016, if Scots voted Yes this September, but stressed: "Negotiations require clarity."
Yesterday, The Herald revealed how a senior colleague of David Cameron had said that a Yes vote would not guarantee independence; that key to the process was the consequent intergovernmental negotiations.
He explained: "A Yes vote in the referendum would be the start of a process, not the end of one - we would start negotiations. But if Alex Salmond made impossible demands, we would not just roll over and agree to everything he wanted. If we could not reach agreement, the status quo would be the default option."
The senior source said the SNP government's 18-month timescale for all negotiations to be completed was "totally unrealistic".
He added: "It would not be a question of defying the wishes of the Scottish people. As the UK Government, we would have a duty to represent the interests of the people of England, Wales and Northern Ireland."
The First Minister responded angrily to the remarks, branding them irresponsible and astonishing, and stressed how they went against the spirit and content of the Edinburgh Agreement.
He said: "Already, it seems they realise they have overreached themselves, given the backlash from ordinary people to their attempts to bully Scotland on the pound.
"Danny Alexander's climbdown, distancing himself from the [senior Coalition source's] comments, suggest they know these tactics are backfiring badly for them, underlined by opinion polls carried out in the immediate aftermath of the currency threat."
Meantime, the No camp sought to intensify the pressure on Mr Salmond over the currency issue, with claims that Mr Salmond - in light of the landmark speech by George Osborne and the united Con-Lib-Lab front ruling out a currency union - had effectively accepted defeat on the issue. He forcefully denied this claim.
Mr Alexander suggested the SNP leader's response to the Chancellor's speech showed "the penny has actually finally dropped about the currency, that a currency union is simply not something that can happen, it would be bad for Scotland and bad for the rest of the UK. And there did seem to be some movement there [from the First Minister] in terms of recognising there are other currency options."
The Chief Secretary insisted the SNP government had now to "move on ... come clean and say what is its alternative."
Alistair Darling, who leads the Better Together campaign, also claimed Mr Salmond had come to the realisation that he could not promise to keep the UK pound and was therefore offering Scotland either a rush to join the euro or a separate unproven currency.
He said: "The referendum choice is clearer after the last 24 hours: a positive vote for the pound or take a gamble on the man without a plan."
There was also pressure elsewhere for Mr Salmond to come up with a Plan B.
Patrick Harvie, the Scottish Green leader, who is part of the Yes camp, said: "We should be looking at the option of an independent currency and deciding whether that's possible or achievable in a timescale by 2016."
Meantime, Mr Salmond, who accused what he branded the "Westminster establishment" of "bluff, bluster and bullying", responded to the suggestion he had no alternative.
He replied: "The fiscal commission working group set out a range of options for monetary policy for an independent Scotland. They're there and have been there for the last year.
"They recommended that the best one was a sterling area and that's the one we're articulating and we will continue to articulate - the plan A."