There is more evidence for the Loch Ness monster than for key parts of the Scottish Government's case for independence, the Chief Secretary to the Treasury has claimed.
The comment was part of an attempt to debunk "myths" put forward by the SNP administration on oil revenues, currency union and wider finances, Danny Alexander told an invited audience of business people in Edinburgh.
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"There is actually one Scottish myth I absolutely cannot and would not be able to disprove," the Highland MP said.
"She's about 40-foot long, publicity-shy and she lives in my constituency, and if anyone here today or any of your families wants to come up to Loch Ness and spend a weekend looking out for her they will be very welcome indeed.
"In short, there is more evidence for the Loch Ness monster than there is for many of the calculations and the claims that have been put forward by the nationalists to support their case for separation."
His remark rounded off a speech in which he said people need a sense of humour to deal with pro-independence claims.
The referendum on September 18 will be a day for the "most serious decision" any Scot will take.
"But increasingly, as the campaign continues, when it comes to some of the statements and assertions made by nationalists you really do need a sense of humour," he said at the Scotland Office.
"Because apparently the same risks that apply to other countries wouldn't apply to an independent Scotland."
Outlining what he calls ludicrous myths, he questioned whether Scotland would be able to bail out banks, given the size of the financial sector north of the border.
And he said First Minister Alex Salmond is being "belligerent" by insisting Scotland would be able to strike a deal to continue using sterling in union with the rest of the UK.
He continued: "There is also the fantastical claim, made in the White Paper, that an independent Scotland would share a third of the UK's institutions and services despite the fact that this is completely unprecedented anywhere in the world.
"This is a claim we have to listen to whenever an institution crops up that the nationalists haven't had time to think about."
He highlighted oil and gas, and the national deficit, as "the more dangerous economic myths" being put forward.
The speech comes on the day HM Revenue and Customs publishes new oil revenue data and before a new UK Government analysis of the economic impact of a Yes vote in the referendum.
Mr Alexander said total UK North Sea revenues last year were £4.7 billion.
Over the past two years, the revenue coming from oil for the UK had been more than £3 billion lower than the Scottish Government's "most cautious" forecasts, he said.
The Scottish Government's Oil and Gas Analytical Bulletin stated that the industry could generate between £41 and £57 billion in tax revenue over the period 2012 to 2017.
Its White Paper on the case for independence put revenue at about £48 billion between 2012/13 and 2017/18.
The figures contrast with forecasts from the UK Government's Office for Budget Responsibility (OBR), which predicted significantly lower oil revenues at about £25 billion over the same period.
Mr Alexander called for a reality check.
"While we should celebrate that our human resources have such potential, we need to be realistic when it comes to our natural resources," he said.
"Our oil reserves are finite and we mustn't be over-optimistic, unproven projections raise false hopes for economic plenty."
After the address, he said pro-independence organisations such as Yes Scotland were "peddling nonsense" to voters.
"Some people are accepting it and so we need to be more on the front foot at debunking some of those myths," he said.
Mr Alexander rejected claims the Better Together campaign against independence was negative and that dry figures on oil revenues were "penetrating" with the public.
"Confirmed supporters of the SNP are known to say anything to try to win the argument," he said.
"What I'm trying to do is bring some facts to the table."
Earlier, Scotland's Finance Secretary, John Swinney, called for the UK Government to spell out details of its share of UK assets if it wanted to maintain a shred of credibility.
"As part of its campaign rhetoric, we know the UK Government talks about Scotland's share of the debt run up by successive Westminster chancellors," he said.
"It cannot be taken seriously if it does not also talk about Scotland's share of assets."
But Mr Alexander refused, adding: "It's an attempt by the SNP completely to distract from the huge gaping holes in their own figures."
Mr Swinney said the Treasury Secretary's speech was "desperate".
"It is the Westminster Government's catastrophic mismanagement of the UK economy which has forced it to borrow almost £200 billion more than it said it would," he said.
"Investment in the North Sea is at record levels and the industry has a great future for many decades to come - but to listen to Danny Alexander and any member of the anti-independence campaign, you would think that wells are running dry all over the North Sea.
"Oil is of course a bonus - not the basis - of Scotland's economy, and as Standard and Poor's recently concluded, even without oil an independent Scotland would qualify for their 'highest economic assessment'.
"This speech is just the latest attempt to frighten the people of Scotland into voting No, and just like all the others it will have the opposite effect.
"Mr Alexander would do well to heed the advice of Professor Leslie Young, who has today described the UK Government's behaviour as 'unworthy of a mature democracy like the UK'.
"As for his bizarre reference to the Loch Ness monster, given his party's current dire poll ratings, Mr Alexander may well find that there are far more sightings of the Nessie in future years than there are of Lib Dems in Scotland."