The highly respected US economist Alan Greenspan has warned against Scottish independence, dismissing Alex Salmond's campaign promises as 'implausible' and saying 'there's no conceivable way' Scotland could share the pound.

The former chairman of the US Federal Reserve accused the Yes campaign of drastically understating the economic damage a Yes vote in Thursday's referendum would cause.

Mr Greenspan, who is known for usually being circumspect, was scathing about Alex Salmond’s oil projections.

He said: ‘Their forecasts are so implausible they really should be dismissed out of hand.’

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Intervention: Alan Greenspan, left, has spoken out against the Scottish independence campaign led by Alex Salmond, right

And he insisted there was no chance of London agreeing to a currency union.

‘There’s no conceivable, credible way the Bank of England is going to sit there as a lender of last resort to a new Scotland,’ Mr Greenspan added.

Washington has become increasingly concerned that its oldest ally is about to be weakened by a Yes vote. President Barack Obama has twice warned against a Yes vote and the White House yesterday said it hoped the UK remained ‘strong, robust and united’.

Yet more high-profile figures made direct appeals to Scots yesterday.

Robert Zoellick, the former deputy secretary of state and World Bank president, said: ‘Like many Americans, and given that my name is Robert Bruce Zoellick, I have an admiration for the Scots, their heritage, and their role in the US and world history.

‘But a break-up of the UK would be a diminution of Britain and a tragedy for the West just at a moment when the US needs strong partners. I strongly suspect it would not work out well for the Scots either.’

Warnings: Other top US figures including Robert Zoellick, left, and John McCain, right, have also flagged up the dangers posed by Scotland splitting from the UK

Plea: David Cameron last night asked Scottish voters to trust in Britain and vowed radical change

Senator John McCain, a former Republican presidential candidate who regards the UK as Washington’s most important military and intelligence partner, said he was reluctant to comment on an internal issue in another country.

But he added: ‘I don’t see how it could be helpful, not just as far as intelligence ties are concerned but to the unique military relationship as well.’

Another big issue for the US would be the uncertainty surrounding the future of Britain’s nuclear deterrent.

American foreign policy experts fear that a Scottish Yes vote would embolden Russia – especially in Ukraine – even as it weakened the UK and thus Europe’s ability to mount a united response.

‘Russia could argue that separatist movements are actually perfectly legitimate, whether in Crimea or eastern Ukraine,’ said Ivo Daalder, former US ambassador to Nato and now president of the Chicago Council on Global Affairs.

‘Of course, a democratic process that takes two years is not the same as an annexation and a fake referendum.’

Yesterday, President Obama’s chief spokesman, Josh Earnest, chose his words with care when asked to comment on Scottish independence.