THE former chairman of Yes Scotland has urged Nicola Sturgeon to open up her party's summer independence campaign to activists from outside the SNP.

It has emerged that the "initiative", revealed by the First Minister to rapturous applause at the SNP conference in March and aimed at building support for leaving the UK, is on course to launch within weeks.

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Dennis Canavan, head of the pro-independence campaign for the 2014 vote, said that a non-party political approach had been vital in winning support and that the SNP drive, which will also re-examine the case put to voters, could learn lessons.

SNP figures including the MP Mhairi Black have called for the summer initiative to be a cross-party effort, while Ms Sturgeon has said she is happy to work with other pro-independence groups to build the case for leaving the UK. However, sources at the Scottish Greens, Radical Independence Campaign, RISE and Women for Independence all said they had not been approached about participating in the SNP-led summer initiative.

With the half way point of summer having already passed, no firm details of what the project entails have emerged, leading to questions over whether the drive had been shelved or delayed as a result of the EU referendum result, which thrust the issue of independence back to the top of the agenda. However, SNP sources denied this was the case and said an announcement would be made "very shortly".

Mr Canavan, asked whether the SNP should now invite other groups to participate, said: "Yes. Obviously the SNP is the main political party which has been campaigning for independence throughout its existence. But nevertheless, I still think there is strength in broadening that campaign to embrace people of different political parties and those who are not part of any political party."

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He also argued that the disbanded Yes Scotland organisation should be resurrected now with Ms Sturgeon saying she is "highly likely" to call a new independence referendum.

Mr Canavan, a former Labour MP who endorsed the SNP at last year's general election, added: "At the last referendum campaign, [the Yes campaign's] breadth was its main strength. It was that broad campaign which managed to increase support for independence from 30 per cent, at the start of the campaign, to 45 per cent at the end. We can build on that. Following the Brexit vote and Trident vote on Monday, there are more and more people, perhaps who voted No, who have been or can be persuaded to vote Yes in the next campaign."

Some SNP figures believe that the summer drive should begin as soon as possible, claiming the prospect of Scotland being taken out of the EU against its will would allow the party to win over "new hearts and minds".

However, the prospect of Scotland quitting the UK to remain in the EU throws up a series of complex questions over issues such as currency, borders and trade, which may prove impossible to comprehensively answer until the details of Britain's post-Brexit relationship with Europe is clear.

An SNP source said: "It's no secret that we're in a situation, post-Brexit, in which it's very important to listen to what people have to say about the case for independence. Brexit has influenced the context in which we'll be undertaking this exercise, and it is obviously a factor. But it will be going ahead, we'll be talking to the whole of Scotland."

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While repeatedly emphasising new independence referendum is "on the table", Ms Sturgeon has said all options to maintain ties with the EU will be explored following last month's vote, which saw more than 60 per cent of Scottish voters opt to remain in the EU but the UK as a whole vote to leave.

Alastair Cameron, Director of pro-UK Scotland in Union campaign group, said: "Breaking away makes even less sense than it did when Scots rejected in 2014. While Nicola Sturgeon may be under pressure to give some hope to hardline nationalists every once in a while, as First Minister she has a bigger responsibility to the majority of Scots who don’t want the division and uncertainty of another referendum and expect her to act in the national interest."