THE SNP should launch an immediate, all-out campaign for independence and be ready to hold a second referendum before 2020, according to the party's former deputy leader Jim Sillars.

The ex-MP has warned the Nationalists could miss a chance to win independence if they fail to secure a clear mandate in next year's Holyrood election.

Writing in today's Herald, Mr Sillars says the economic case for independence must not be based on the promise of a currency union with the rest of the UK, as it was last year, but on a plan to create a new Scottish currency.

He reveals in a new book, to be published on the anniversary of the referendum, that a majority on the board of Yes Scotland, the official pro-independence campaign, wanted to present an alternative to Alex Salmond's plan to share the pound in a "sterling zone".

Mr Sillars' comments amount to a complete rejection of Mr Salmond's central economic policy and of the cautious approach adopted by his successor, Nicola Sturgeon, to a second referendum.

It comes as a new poll showed Scotland faces the prospect of leaving the EU against the wishes of a majority of Scots - one of a limited number of scenarios the First Ministers says could trigger an early re-run of last September's vote.

Throughout the referendum campaign, Mr Salmond claimed an independent Scotland would share the UK pound, arguing it was in the interests of both countries.

He continued to make the case after it was rejected by Chancellor George Osborne, backed by his opposite numbers in Labour and the Lib Dems, who said it would be too risky for the rest of the UK.

The Yes campaign insisted Mr Osborne was bluffing.

In his book, In Place of Failure, Making it Yes Next Time...Soon, Mr Sillars argues the policy allowed the pro-UK campaigners to create uncertainty in the minds of voters and was "a gift to the No side".

He tells how he contacted members of the Yes Scotland board to urge them to develop a Plan B on currency.

He writes: "Most of them agreed and at one meeting of the advisory board a majority wanted an alternative plan.

"This fact has never been made public, primarily because that majority was persuaded that to declare for a plan B would be seen as the Yes side being split.

"Common sense gave way to the imperative of supporting the White Paper.

"So the whole Yes Scotland movement found itself tied to an SNP policy which many - very many - did not agree with."

Among those in favour were former Labour MP and Independent MSP Dennis Canavan, who chaired the board, and Patrick Harvie, the co-convener of the Scottish Greens.

The most powerful voice on the board, however, was Ms Sturgeon, who had been responsible for drawing up the White Paper, Scotland's Future, her party's all-important blueprint for how an independent Scotland would function.

Like Mr Salmond, Mr Sillars, a former MP who was married to the later Margo MacDonald, believes the UK Government would have agreed to a currency union in the event of a Yes vote.

But he still opposed the plan, arguing it would work in the interests of the dominant partner, the UK, with Scotland having "to approach London as a supplicant" when it came to negotiating on macro-economic policy.

He writes: "The only conclusion we can reach from that scarring experience is that there can be no more talk of currency union.

"The independence movement must opt for the most obvious and sensible alternative: our own currency."

Since the referendum, Mr Salmond has argued the pro-UK parties' offer of further devolution was the decisive factor.

However leading Nationalists are increasingly questioning his currency plan.

In a newspaper column yesterday, the former MSP Andrew Wilson said there was "no doubt" that further work was needed on the economic case for independence, including the question of currency.

In his article for The Herald today, Mr Sillars calls on the SNP to mount a "sustained independence campaign" before considering a date for another referendum.

But he says if Ms Sturgeon does not make a clear commitment to independence in next year's Holyrood election, "any idea of a referendum over the next four years disappears".

He adds: "What is required now, based on an analysis of what went wrong in 2014, is a campaign for independence, not devolved power.

"That is the horse, and the cart, which is the referendum, should be ready to roll when the time is right, but before 2020."

An SNP spokesman said: "Jim was a powerful voice in the Yes campaign.

"But the reality is the No campaign were intent on speading fear and uncertainty at every turn and would have done so whatever options had been proposed on currency and Europe."