Patrick Harvie claimed that while it was unlikely that Boris Johnson would shift his opposition to granting Holyrood the legal powers required to hold such a vote in the short term, continuing Westminster opposition would only drive more undecided voters into the Yes camp.

The Scottish Greens’ co-leader, who played a key role in the 2014 independence campaign, played down opposition from some pro-independence quarters to Nicola Sturgeon’s speech on Friday, in which the First Minister accepted an indyref2 may not happen until after the 2021 Holyrood elections.

“The issue is, what is the Scottish Government going to do to try and achieve the progress that we need toward putting Scotland’s future back in the hands of the people that live here?” said Harvie.

Patrick Harvie takes a selfie with supporters at the Brexit day launch of the Greens' campaign for Scotland to re-join the EU in Glasgow's The Barras. Picture: John Devlin

“I’m not going to try and pretend that’s going to be easily achieved, or that Boris Johnson is going to suddenly wake up and tomorrow and decide he was wrong.

“It’s going to be a tough argument – but there is a point of principle. I don’t think it’s OK to just give up and roll over and wait for Brexit to damage to our society and our economy.”

He continued: “My best guess is the Tories will reach a point of recognition. The longer they hold out and say: ‘You’re not allowed to make this choice’, the stronger the support for independence will become.

“They will recognise that if they have any chance at all of persuading people not to opt for a positive future, as an independent member of the European Union, they are going to have allow that question to come sooner or later. The longer they leave it, the more determined people will become.”

The SNP leader announced in a speech to party members last week plans to set up a new “constitutional convention” to establish a new Claim of Right and forge a broader campaign for a Yes vote.

But the speech was branded “utterly predictable and entirely wrongheaded” by pro-Union leaders.

The Greens, meanwhile, used an event in Glasgow on Friday to launch a renewed campaign in favour of independence, with the eventual aim of a new Scottish state joining the EU.

Ska Keller, co-president of the Green group of MEPs in the European Parliament, used a speech in Glasgow to tell supporters that Scotland would be welcomed as a member state.

Outlining his continued push for an indyref2, Harvie added: “Greens are used to arguing against the odds and maintaining a principled position even if others are saying no.

“It is important that we do continue to assert the right of Scotland to decide the time-scale of the transition period. Every other member state has had a say, the UK has had a say. Scotland has tried to have a say and been ignored.

“I think there is a principled argument to be made, that during the transition period, Scotland should have a say.”

The First Minister has previously spoken of her desire to keep the “gold standard” of the 2012 Edinburgh Agreement – which laid down the rules for the 2014 referendum – but has so far refused to rule out a consultative ballot.

“Should the UK government continue to deny Scotland’s right to choose, we may reach the point where it is necessary for this issue to be tested, she said last week. I am not ruling that out.”

But the Greens list MSP maintained that calls for such a vote should not be heeded.

“I don’t think anyone should be rushing towards that as an option,” he added.

“I think it’s far better if we can have this choice on a democratic basis without going to court, because I think that’s the best way to make sure both sides of the argument accept the rules and the legitimacy of the outcome.

“If at some point it becomes utterly impossible and the UK becomes intransigent, even after 2021, well possibly you might have to look at that. But it’s by no means an option we should be enthusiastic about.”

Don’t lose faith in indyref2, Patrick Harvie urges SNP