THE former SNP MSP who was instrumental to the party achieving power is to turn his back on it after 55 years to support a new pro-independence group for Holyrood.

SNP legend Dave Thompson said he hoped the Alliance for Independence would “max the Yes” and win up to 24 seats on the regional list system for a decisive Nationalist majority in 2021.

However SNP cabinet secretary Mike Russell said his old colleague was making a mistake, and the party warned talk of splitting the SNP vote would be “music to the ears of the Unionists”.

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The move comes amid an increasingly rancorous debate in the SNP about whether the party should contest list seats it appears unlikely to win or let another party mop up pro-Yes votes.

SNP MP Kenny MacAskill last week said that the SNP standing on both the constituency and top-up lists “just doesn’t work”.

A Panelbase poll also found up a quarter of Scots might back a pro-independence party fronted by former First Minister Alex Salmond.

Advocates of a new party argue it would maximise the number of pro-independence MSPs and force Boris Johnson to grant a second independence referendum.

However it could also be seen as gaming the electoral system and a further manifestation of the divisive personal feud between Nicola Sturgeon and Mr Salmond.

Mr Thompson is an iconic figure in the SNP as it was his quick-thinking demand for a recount that led to the party winning the 2007 election by a single seat over Labour.

He served as a Highlands & Islands list MSP for four years, then as MSP for Skye, Lochaber and Badenoch from 2011 to 2016.

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Mr Thompson, 70, told the Daily Record he was on the steering group of Alliance for Independence (AFI), which would launch soon.

The umbrella group is intended to bring together smaller pro-independence parties under one banner to stand to maximise Yes MSPs on the the eight regional lists, where candidates can get elected with as little as 6 per cent of the vote.

Because the SNP did so well in the constituency vote in 2016 - winning 59 of 73 seats - it won only four of the 56 top-up seats on the list system.

By only standing on the list, the Alliance would try to mop SNP votes it says would otherwise be wasted in order to maximise the number of pro-Yes MSPs and secure Indyref2.

However the list system is notoriously hard to predict.

The SNP's outright majority of 2011 also relied heavily on the lists, with 16 of its 69 MSPs elected in this way.  

Mr Thompson said: “Every regional list vote for the SNP will have no impact. It will achieve nothing. Whereas if a lot of these votes came to AFI we can garner a lot of MSPs.

“We are looking at anything between 8 and 24 MSPs.”

“As soon as we launch, and I formally join the Alliance, I will leave the SNP, which won’t be easy for me, as I have been in the SNP since 1965.”

He said the Alliance would support the SNP in first-past-the-post seats and its members would only have to agree with independence.

He said: “On everything else they can vote according to their own party programme, or according to their own views and conscience.

“I do believe, tactically, we can gain far more independence-supporting seats by having a different vote on the regional lists. 

“If I can achieve that more quickly by voting SNP in the constituency and voting Alliance on the list, that’s what I will do. I do believe that’s the best way.”

On whether he would like to see former SNP First Minister Alex Salmond join the Alliance, he replied: “We would welcome anyone and certainly high profile people would be good.”

SNP Constitution Secretary Mike Russell said his former colleague was “mistaken”.

He told BBC Radio Scotland: “Anybody is entitled to join or vote for any party they wish.

“I know Dave very well, I’ve worked very well with him. 

“I think he’s mistaken in his analysis. I believe that in order to get independence we need a unified movement.

“The SNP is the key part, or a key part, of that movement. I’ve been a member of the SNP now for 40 years and I’m certainly not changing my view.” 

Shadow Scottish Secretary Ian Murray, the Labour MP for Edinburgh South, said: “The anti-UK movement is trying to find new ways to make next year’s election about a divisive second independence referendum.

“Next year’s election is too important to be dominated by SNP infighting over independence. It should be about the future of our hospitals, schools and jobs.”

An SNP spokesman said: “Talk of splitting the SNP vote will be music to the ears of the Unionists. Those seeking to game the Holyrood proportional electoral system are putting at risk the SNP’s progress.”