Former SNP leader Gordon Wilson has said Scotland should declare independence if the rest of the UK votes to leave the European Union against its wishes.

Mr Wilson acknowledged that a "confirmatory" referendum may be "politically desirable".

But he said the Scottish government would have a mandate to retain membership of the EU by leaving the UK.

Prime Minister David Cameron has promised an "in/out" vote on the EU if the Conservative party wins an overall majority at the general election on 7 May.

In the event of a hung parliament, Mr Wilson, who led the SNP throughout the 1980s, argued that the package of extra powers for the Scottish Parliament, designed by the Smith Commission and endorsed by the three main UK parties, should be ditched in favour of a more radical approach.

In a paper examining the prospects for independence six months after it was rejected by 55% to 45% in a referendum, Mr Wilson said the SNP "should start afresh and negotiate fiscal autonomy," if it were in a position of influence at Westminster following the election.

Polls suggest the SNP could hold the balance of power after the poll, perhaps returning 50 or more MPs to a House of Commons where neither of the two main parties had an overall majority.

This, argued Mr Wilson, would present an opportunity to reject the "harmful" and "incoherent" proposals of Lord Smith which he claims amount to "a poison pill".

"More powers and less money is a recipe for frustration and anger," he wrote.

Instead, he suggested Scotland should emulate the Isle of Man "which has its own fiscal system and makes payment to London for foreign affairs, defence, national insurance and other shared costs."

"Devolution has run its course and is no longer viable," insisted Mr Wilson.

'Public demand'

But he warned the current SNP leadership against being "carried away" by "political power-play" in the event of negotiations at Westminster after a hung parliament.

"They must vote the way that benefits Scotland and that they can justify on the ground," he said.

He was also strongly critical of the party's campaign during the 2014 referendum, saying it "failed to win the economic arguments."

"They were too timid and failed to present a case on the currency, a central bank and fiscal policy that was credible. That must not happen again," he wrote, in the paper entitled "The Referendum Six Months On".

The former MP for Dundee East rejected the suggestion that "The Vow" delivered by the leaders of the Tories, Labour and the Liberal Democrats two days before the referendum was decisive, calling it "meaningless, vague and ill-defined."

The battle for independence is not over for a generation said Mr Wilson, who argued that another referendum should be held in the next five to 20 years but only when there is "sustained support of over 55% and substantial public demand."

Economic arguments

He proposed that an "Independence Convention" be held on 19 September, a year to the date that the result of the referendum was published, to consider the "strategies needed" to win a second plebiscite.

An SNP spokesman said: "As the first minister has made clear, the people of Scotland will decide when the next referendum is held. Scotland can only become independent through a majority vote in a referendum on Scottish independence."

Liberal Democrat MP for Edinburgh West Mike Crockart said: "Gordon Wilson is right that the SNP lost the "economic arguments" in the referendum campaign.

"They need to accept that and move on. While the SNP still has its eye off the ball on day-to-day issues, thousands of people are waiting hours to be seen at A&E units or struggling to find a place at a college hit by cuts.

"During the referendum campaign, Nicola Sturgeon said that the "once in a generation" vote meant the issue was decided for 15 years, she needs to come clean on whether she now agrees with her former party leader."

A spokesman for Scottish Labour said Scots would be "concerned" that a senior SNP figure "is welcoming further austerity as a means to achieving a political end".

He added: "Scotland cannot afford another five years of the Tories, whose plan would take us back 1930s levels of public spending, before there even was an NHS.

"Meanwhile, SNP plans for Scottish only taxes to support Scottish spending will end the Barnett formula, cost Scotland billions and end shared UK pension, meaning austerity max for Scotland.

"Neither option is best for Scotland."