NICOLA Sturgeon should shelve an independence referendum until after the next Holyrood election in 2021 when she could win an “indisputable mandate” for such a vote, a former SNP minister has said.

Marco Biagi said the SNP had to "let time pass" and rebuild the campaign for independence in the wake of the party's General Election losses.

Biagi, who stepped down as an MSP last year, said that "Brexit will help the cause then, when its effects are tangible".

His remarks came after Sturgeon admitted that SNP plans for a second referendum were "undoubtedly" a factor in the loss of 21 Westminster seats.

The First Minister said the electoral setback was something she would upon "reflect carefully".

Biagi said losses to Unionist parties showed that the SNP's current timescale for holding a referendum was "giving their opponents a stick to beat them with".

Former SNP cabinet minister Alex Neil and the party's ex-deputy leader Jim Sillars have also said it is time to go back to the drawing board and remake the independence case.

Biagi said the SNP had to regain the "energy and acumen" he claimed it showed ahead of its landslide win in 2011, that led to the referendum.

Biagi, a former local government minister, said the party needed to "show the same sort of governing energy and acumen that built the party's support between 2007 and 2011. Use the new Scotland Act powers to the full while demonstrating their limitations. Let time pass. Let need build. Brexit will help the cause then, when its effects are tangible. Then win a Holyrood election and hold a referendum quickly thereafter with an indisputable mandate.”

Biagi, who is now an academic, also warned that any attempt to hold an referendum without the agreement of the UK Government would be blocked by the courts.

He said: "The SNP clearly has a democratic mandate for an independence referendum from the last Holyrood election manifesto, but support from the public for the vote hasn't materialised. Polls even show a significant minority of those who would vote Yes in any referendum don't actually want such a referendum to be held at the moment. And the real problem is that by keeping what is - rightly or wrongly - seen as an immediate referendum on the table, the SNP is giving their opponents a stick to beat them with.

"That might be fine if there was a prospect of the referendum happening, but there isn't. The UK Government is dead against. Any attempt to hold a referendum without an equivalent of the Edinburgh Agreement would end up in the Supreme Court, and based on recent decisions would be defeated there."

Meanwhile, former SNP deputy leader Jim Sillars said the SNP had to relaunch the party's independence prospectus. He said the SNP had to launch a major 'educational campaign' to promote the case for independence.

He added: "Of course it's a setback. The party of independence can't lose that many seats without it being a setback. There are four years to go to the next great test of the 2021 Scottish Parliament elections. While four years is a long time the repair work has to start now. I hope that Yes supporters do not despair as it's possible to fix the SNP.

"We have to conduct a major independence educational campaign to win an absolute majority in 2021 to give us the basis for another referendum. The policy work on why we lost in 2014 has not been done and we do need to talk about the mistakes that were made in 2014."

Alex Neil, Scotland's former SNP health secretary, said the SNP had to review the way it made the case for independence in the light of Brexit. The MSP said the party had to take account of changes this would mean for how it promoted independence.

He said: "The immediate priority has to be to have a review of the independence referendum strategy to focus on issues around the Brexit negotiations. We've got to demand that any deal is good for Scotland and we have got to make sure that the powers are transferred to Scotland. But it's all got to be based on an agenda that advances a progressive social and economic agenda.The SNP could recover quite quickly provided we reboot the strategy. We could turn it around by the time of the next Scottish Parliament elections."

The SNP said the party did not have anything to add to the post-election statement Sturgeon made. Reacting to the result, Sturgeon said: "Undoubtedly the issue of an independence referendum was a factor in this election result, but I think there were other factors in this election result as well." Sturgeon said Brexit, a late surge in support for UK Labour leader Jeremy Corbyn and tactical voting were some of the other factors which had contributed to the results.