NICOLA Sturgeon will win a second independence referendum within two years if the UK government fails to meet her demand for a soft Brexit for Scotland, her predecessor has said.

Alex Salmond said the First Minister would have no compunction in going to the country even if the polls showed support for a Yes vote still at their current level.

Ms Sturgeon will unveil her long-awaited plan for a special Scottish deal for Brexit on Tuesday.

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Titled “Scotland’s Place in Europe”, the document is expected to lay out a range of possible options for maintaining the country’s access to the EU single market after Brexit.

It will argue the best option for the whole UK is staying in the single market, although this would involve free movement of people, which Theresa May has ruled out.

The paper will also suggest Scotland-only options for staying in the single market, including Norway-style membership of the European Economic Area, but this would require the “substantial” devolution of new powers over immigration, business regulation and employment.

Chancellor Philip Hammond has dismissed a Scotland-only deal as “not realistic”.

Writing in the Herald’s sister paper, the Sunday Herald, Ms Sturgeon said she still believed the best future for Scotland was as an independent EU state, but “in the spirit of compromise”, her government had examined a range of other options.

However she warned: “Implementing our proposals will not be straightforward. It will require co-operation, imagination and flexibility.

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“If our interests cannot be protected… or are indeed brushed aside by the UK government, then the people of Scotland should have the option of considering independence.”

In TV interviews yesterday, Mr Salmond, the SNP foreign affairs spokesman, suggested single market membership would be key to holding and winning a second referendum.

He told BBC Sunday Politics Scotland: "The last time, when I was First Minister and embarked on this process, support for independence was 28 per cent.

“After two years we ended up at 45 per cent. So I don't think Nicola Sturgeon would have any compunction about calling an independence referendum with support in the mid-40s."

He went on: "In a situation where the UK Government was determined... to sever our ties as a thousand-year-old European nation, then I think that would bring many people who were previously sceptical about independence to the Yes side."

Asked, "And you think you would win?" he replied: "Yes I do".

Adam Tomkins, the Scottish Conservatives’ constitution spokesman, said: "Scots don't want to go through another divisive independence referendum, but Alex Salmond repeated the SNP's threat to press ahead regardless.

"As we begin negotiations to leave the EU, we need a Scottish Government that ends the sabre-rattling, and focuses instead on the national interest, not its own political interest.

"As she publishes her paper this week, it is vital that Nicola Sturgeon shows she is prepared to be a First Minister for Scotland, not a First Minister for the SNP."

Scottish Labour's Europe spokesman Lewis Macdonald said: "Alex Salmond gave the game away when he made it clear that the Nationalists are only really interested in finding an excuse to impose another referendum on the people of Scotland.

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"The UK is Scotland's most important market. The SNP's own figures confirm remaining part of the UK single market is more important for Scotland's economy than even being in the EU.” Scottish Secretary David Mundell has said the UK government will consider the SNP paper, but also that it would need to be “credible and backed up by evidence” to outweigh Westminster’s own research that Brexit should be the same across the whole of the UK.