ALEX Salmond has dismissed Treasury demands that he withdraw his threat to renege on an independent Scotland's debt obligations if the Scottish Government was unable to secure a currency union with the UK.

The First Minister's brush-off came as Labour and trade union leaderships prepare to step up their involvement in the No campaign. Ed Miliband, the party leader, is due in Scotland on the campaign trail towards the end of the week.

His colleague Margaret Curran, the shadow Scottish secretary, writing on the Labour List website, warned the party could not afford to be complacent as the Yes camp targets Labour supporters.

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"The SNP are trying to win over Labour voters and we have to be out there fighting for every last one. We've seen in elections before what can happen if we're complacent. The people of Scotland rightly expect a lot from the Labour Party, and in the next two weeks we have to show them that we mean business," she said.

Earlier, Danny Alexander, the Chief Secretary to the Treasury, wrote to Mr Salmond, urging his ministers to roll back on their "bogus" debt threat. His Liberal Democrat colleague, Willie Rennie, the Scottish party leader, said a decision to default on debt "would trash our reputation across the globe".

But Mr Salmond said he would "certainly not" withdraw the threat, insisting Scotland could not default on a debt it was not legally liable for and would have no moral obligation to pay without a share of Bank of England assets, ie sharing the UK pound.

In a separate development, Mr Salmond responded to David Cameron's admission that he was nervous about this month's ­referendum result, noting: "He is right to be so with the most recent polls putting a Yes vote within touching distance and hundreds of thousands of Labour voters set to back independence as momentum is now firmly with the Yes campaign."

Today, Alistair Darling, leader of the Better Together campaign, is to launch a new set of campaign posters in Greenock, when he is expected to say: "One million jobs rely on our links with the UK. I want a million more; not to put the million we currently have at risk."

Other developments included:

l The Centre for Economics and Business Research estimated the start-up costs for an independent Scotland to be nearly £2.5bn with another £2bn costs because of the need for new currency arrangements, 10 times the SNP's preferred estimate;

l The SNP called on Alistair Darling to "come clean" on his role in a deal between the previous Labour government, HBOS and UK financial regulators, which allegedly resulted in a cover-up of hundreds of millions of pounds of public money being secretly used and

l A poll showed 44 per cent of people in Britain were against a post-independence currency union with 22 per cent in favour, while the Scottish breakdown showed 21 per cent were against while 60 per cent were in favour.