David Martin, the former Labour MEP, said last night he has received “unequivocal assurances” from the Scottish Government that it will be wholly independent of the SNP, as he addressed an audience in Edinburgh.

Pro-Union parties have already pledged to boycott the body amid concerns it is a “Nationalist stitch-up” to secure a second referendum on independence, although Martin said this will not derail its work.

The 120-strong body will get under way in October with a series of meetings until next spring with a broad remit to respond to the challenges of Brexit and information needed to make “informed choices” about Scotland’s future.

“This assembly is going to try and change the tone of the debate in Scotland – it’s about a respectful discussion about where Scotland should be in future,” said Mr Martin who will co-chair the body.

“Given some of the coverage of this, I want to emphasise this will be completely independent of government. Of course the government will set it up and it gives it credibility that the government has set it up, because that gives it status.

“But once established I have had unequivocal assurances from the minister responsible, from Mike Russell, that the two co-chairs will be independent and will manage the process themselves – no external political interference.”

Mr Martin, speaking an Electoral Reform Society event, said the assembly’s remit is “quite broad” and he will be seeking to the “narrow the question down” during its deliberations.

But he added: “It is not going to decide whether Scotland should have a second referendum because that’s a matter for the Scottish Parliament and the Scottish Government. They’ve already legislated for that. It’s not a matter about when the timing of that will be, because that’s a matter for the Scottish Government and the government to negotiate.

“It’s not even a matter for what the question will be, it’s not also a matter of whether Scotland should be an independent country.”

The Tories and Liberal Democrats at Holyrood, along with the Scotland in Union organisation have already announced they won’t take part and urged supporters to boycott the Assembly, but Mr Martin said he hoped “they will changed their minds”.

Nationalist MP Joanna Cherry hardened attitudes among pro-union parties recently when she claimed it would help pave the way to independence, but sought to clarify her position last night.

“It will come as no surprise to anyone that as an SNP MP I hope that Scotland will be an independent country, so from that perspective I see engagement with issues that need to be tackled in Scotland and discussion about them as a way that might move us forward towards independence,” she said.

“But other people are coming to the Citizens’ Assembly from a different perspective and will see it differently and there are many issues that we could discuss that usefully that could unite those of us who want independence and those of us who don’t.”

The Edinburgh South West MP has been a long-term campaigner for a Citizens’ Assembly and said it should help defuse the “tribal and political affiliations” of Scottish politics.

Scotland’s new Citizens’ Assembly to ‘change tone of debate’