Jim Sillars, who served as deputy leader between 1991-1992, claimed the current leader has miscalculated her independence campaign to believe every Scottish voter who backed Remain wants to leave the United Kingdom.
The 79-year-old insisted it would be a “mistake” for Ms Sturgeon to build her campaign around membership of the European Union because it alienates Scottish Leave voters.
Mr Sillars, who campaigned for Brexit, told BBC Radio 4’s Analysis programme: “I, for example, could not vote yes if on the ballot paper it said, ‘We wish the Scottish state to be a member of the European Union’, and I am not alone in that.
Jim Sillars ripped into Nicola Sturgeon's 'fanciful' independence hopes
It's a mistake by Sturgeon is to believe Scots who voted Remain would then vote to go back into the EU
“That means Ruth Davidson, the leader of the Tory party, and all of the Tories who voted to remain would, in fact, vote to leave the United Kingdom and take a Scottish state into the European Union – I think that’s fantasy.”
Mr Sillars was not alone in his criticism for Ms Sturgeon, former SNP leader Gordon Wilson also attacked her and the party’s “obsession” at attempting to deliver the “non-deliverable” promise of EU membership.
Mr Wilson, who led the SNP between 1979 and 1990, said they were unprepared for the “vacuum and drift” over how to lead a second drive for separation.
“The Scottish National Party must exercise extreme care if, as it appears, it is caught in a political vortex and cannot avoid holding a second referendum on independence at the worst possible time.
“For many supporters, that is not the only problem. It is the seeming unpreparedness, almost as if the party hierarchy is giving priority to the local government elections.
“When are we to be given the answers on the currency, budget and trade? Who is to lead the campaign and how is it to be financed.”
An SNP spokesman said it was “fully focused on protecting Scotland from the disastrous impact of a ‘hard’ Tory Brexit”.
He added: “Instead of rejoicing in the downturn affecting the North Sea oil industry and the jobs affected by it, the Scottish Tories should be lobbying for support for the industry in this week’s UK Budget.”
Ms Sturgeon is expected to step up her threats to break up Britain in a major announcement at the SNP’s spring conference next week.
Sturgeon's top moments Fri, December 9, 2016
Scotland's First Minister Nicola Sturgeon in pictures.
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Nicola Sturgeon visits Glaxo Smith Kline.
She is due to argue another poll would not only be “legitimate” but “almost necessary” if her demands for a bespoke Brexit deal are not met by the UK Government.
Ms Davidson has refused to speculate on whether a second referendum should be held after Brexit, but argued Ms Stugeon had broken the terms of the Edinburgh Agreement, the deal signed by David Cameron and Alex Salmond to stage the last referendum, in which both sides promised to respect the final result.
“If she does try and call one, I think she’s going to take a massive hit because Scots don’t want to be dragged back there,” the Scottish Tory leader said.
“She moves against the popular opinion in Scotland and stops acting as a First Minister and acts solely as a leader of the SNP then I think she will pay a heavy price for it.”