Alex Salmond and Nicola Sturgeon have launched an emotional new bid to win support for "Scotland's hour" of independence amid signs that uncertainty over currency is nudging undecided voters towards a No vote.

Senior Scottish Government sources acknowledged that a conscious effort is being made to create a "sense of moment" around the referendum, now less than 200 days away.

The First Minister will use an emotional speech in London tonight to describe the vote as an "opportunity" "for everyone in Scotland ... when they stand in the polling booth knowing they are helping to shape their country's future.

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"Let's call it Scotland's Hour. Because on that moment, and from then on, Scotland's future will be in Scotland's hands."

His cry will be seen as a reference to his literary hero Robert Burns, who in his poem Scots Wha Hae imagined Robert The Bruce telling his troops before the Battle Of Bannockburn "Now's the day and now's the hour".

It is also likely to be seen by its English audience as a nod to Prime Minister Winston Churchill's Their Finest Hour speech delivered in June 1940 to the House of Commons, just a few hundred yards from where Mr Salmond will be speaking.

Telling MPs that he was confident of victory in the Second World War, despite the fall of France, Mr Churchill said: "Let us therefore brace ourselves to our duties ... that if the British Empire and its Commonwealth last for a thousand years, men will still say, 'This was their finest hour'."

Yesterday Ms Sturgeon also sought to build a sense of history around the September 18 vote.

She used a speech to the Scottish Council for Development and Industry in Glasgow to claim the future of Scotland would "for just a few short hours" be in the hands of its people on polling day and said that voting No would be "a difficult thing to explain to future generations".

A Scottish Government source insisted there would be no retreat from the "hard-headed" arguments for independence, including on currency. "But allied to that will be an appeal to the heart as well", he said.

An Ipsos Mori poll yesterday for STV News found that 34% of undecided voters said they were more likely to vote No after the main UK parties ruled out Mr Salmond's plan for an independent Scotland to share the pound in a currency union with the rest of the UK.

By comparison, just 16% felt they were more likely to vote Yes following the announcement three weeks ago.

Meanwhile, Ladbrokes has lengthened the odds of a Yes vote to 11/2, the longest price offered by the bookmaker since the campaign started.

Ms Sturgeon told the business leaders: "Think how you'll feel not just in 10 or 20 years when you are telling your grandkids how you voted, think how you'll feel on the morning after."

During his speech tonight Mr Salmond will also attack the militaristic language employed by the UK Government in recent weeks.

He will criticise the likening - by Government sources - of Chancellor George Osborne's currency union announcement to the Dambusters.

The First Minister will say: "We were love-bombed from a distance by David Cameron, then dive-bombed at close range by George Osborne."

He will add that he believes Mr Osborne's speech on sterling "will come to be seen as a monumental error".

A Better Together spokesman said: "This speech is a spectacular own goal from the First Minister. Not content with telling every expert or employer they are wrong, Alex Salmond now wants to pretend that a poll showing support for separation falling because of his failure to tell us what will replace the pound never happened."