The former cabinet minister, Alex Neil, made the call on the eve of the SNP conference amid frustration from grassroots supporters that more isn’t being done by the SNP leader to prepare for another crack at independence.

IN PICTURES: All Under One Banner Edinburgh rally

Ms Sturgeon will tell SNP delegates in Glasgow that the Brexit “shambles” makes the independence case “more compelling than ever”. But as an estimated 20,000 independence campaigners marched through Edinburgh, activists warned that time was running out to stage an independence referendum and accused the SNP leadership of not doing enough to make the case for splitting up the UK.

Pro-independence supporters march through Edinburgh, during the All Under One Banner march. Picture: Jane Barlow/PA Wire.

Poll: Half of Scots would vote for Scottish independence after Brexit

Yesterday’s “All Under One Banner” demonstration ended with a mass rally at Holyrood Park in defiance of Historic Environment Scotland (HES), the body responsible for the ground.

Speaking ahead of today’s conference, Neil accepted that the need for a Section 30 order which would see the UK government transfer the power to hold an independence vote from Westminster to Holyrood meant that a second plebiscite would have to wait until after the 2021 Scottish election. Prime Minister Theresa May has stuck to her line that “now is not the time” for indyref2, pointing out that the SNP had promised it would be a once in a generation vote.

But Neil called on Sturgeon to start working on a fresh independence white paper in order to rally voters behind the SNP in the run-up to 2021 so that the party secures a pro-independence majority. Neil argued that a detailed independence plan which updated Alex Salmond’s 2013 White Paper would encourage the 45 per cent of the electorate who supported independence in 2014 to get behind the SNP.

“Next year she should publish a new white paper for independence and publish a bill so that when we reach 2021 we are ready to go,” said Neil. “We have got to build up momentum and engage in a persuasion exercise. That way there will be no delay. It will also get 45 per cent of voters behind us going into the election, which would give us a majority in the Scottish Parliament.”

Earlier in the year, Sturgeon said she intended to use autumn conference to outline her indyref2 plans on the basis that the detail of the Brexit deal would be known. But with the outcome of the EU withdrawal negotiations still unclear, Sturgeon is highly unlikely to make a definitive announcement. With grassroots activists agitating for a second poll, one of her challenges will be to persuade them that she is not dithering. A further complication is that Alex Salmond’s shadow will loom over the conference. Although the former first minister has resigned from the party following the sexual misconduct claims made against him, there are many supporters who favour his more gung-ho approach to indyref2.

Yesterday Jonathon Shafi, co-founder of the Radical Independence Campaign, warned more needed to be done to give the activists who flocked to the party in the wake of the 2014 defeat a sense of direction.

“When it comes to independence, time is running out to utilise the mandate won at the 2016 Holyrood election,” said Shafi, who marched in Edinburgh yesterday and is speaking at the conference.

“In addition, the SNP leadership have not been developing the pro-independence case and taking it into the living rooms of Scots. This despite the context being the acute crisis of the British state as a result of Brexit.” Shafi is disappointed that discussion of independence strategy and the SNP’s economic independence blueprint – Andrew Wilson’s Growth Commission – are not on the official agenda. Those on the left of the party have been highly critical of Wilson’s document claiming its debt reduction strategy means many more years of austerity.

Shafi said: “The Growth Commission adds a further problem. Since it has been widely rejected, and is certainly not being held aloft as a rallying call, there is no economic programme being advocated by the SNP that is ready to go. This further complicates calling for a referendum at short notice, reducing tactical flexibility. A crossroads is approaching. Nicola Sturgeon must speak to, not avoid, the substantial post-2014 base. Today’s huge demonstration shows that mass mobilisation is possible. The question now is whether the SNP leadership are willing to give a lead by making the independence case, laying out an inspiring vision and building momentum. That doesn’t mean calling a referendum tomorrow, but it does mean arming activists with a sense of direction.”

But in an illustration of the dilemma that Sturgeon faces, some in the SNP were urging patience. Former SNP deputy leader Jim Sillars said: “To go at things like a bull at a gate, in my view, is only inviting defeat because we are not ready.”

Sillars was also of the view that not enough was being done to prepare the ground for another vote, arguing that an over-arching pro-independence organisation should be set up to look at the implications of Brexit.

“The Yes movement needs to get together into a single Yes organisation with research capabilities at the centre to examine very clearly what the Brexit treaty is and what it will be and then examine how it starts to work out in practice,” said Sillars.

Yesterday, Sturgeon said the conference will set out why the full powers of independence are essential to fulfil Scotland’s potential.

The SNP leader said: “The shambles of Brexit makes the case for independence more compelling than ever – with Westminster ignoring Scotland’s voice and interests and undermining devolution with a power grab on the Scottish Parliament.”

Last night the Tories said it was a “shame” marchers had ended up at Holyrood Park and said they should respect the law. HES had said events of a political nature should not be held there, but then said its priority was to facilitate the event safely. Police said council estimates put the crowd at 20,000. Gary J Kelly of All Under One Banner said 100,000 took part.

Scottish independence marchers ramp up referendum pressure on SNP