The SNP has come out fighting over the issue of how independence supporters should cast their second vote.

The party, which is set for another landslide at the Holyrood elections, got tough following a study by Professor John Curtice, one of Scotland’s top political scientists, which suggested that independence supporters risk “wasting” their regional list - or second - vote unless they back another pro-independence party such as the Greens.

In the report commissioned by the Electoral Reform Society (ERS) Scotland, Curtice collated and analysed recent opinion polls and projected that the SNP will win all but three constituencies, but - because the electoral system makes it more difficult for a party which dominates in the constituency vote to get list MSPs elected - return only two regional list MSPs.

The Strathclyde University professor said if an average of recent polls are correct the SNP is set to secure a majority through constituency seats alone, and that “many a list vote for the SNP would be wasted”.

But suggestions that independence supporters should therefore give their regional vote to another party are dismissed today by SNP MP Mhairi Black as “wrong" and "misguided".

However, Curtice’s projection has now been backed by academics at the University of Aberdeen and the University of Stirling, as well as the head of the Scottish arm of leading pollsters Ipsos MORI.

Speaking to the Sunday Herald yesterday, Curtice underlined the conclusions he reached in the ERS report, saying: “Given the character of the polls – leaving aside the caveats about the accuracy of opinion polls on the list vote – some Yes voters may think it’s a waste of time voting for the SNP on the list vote...I’m simply pointing out there may be a tactical dilemma here, although it’s one with risks associated with it, but you may be inclined to the view that it makes more sense to vote for the Greens than the SNP on the list vote because it might make more impact in terms of delivering more MSPs in favour of independence.

“We know the SNP are sensitive about this...I understand where they’re coming from...They wish to try and stop people from wandering off to vote for the Greens. If the opinion polls are right it is undoubtedly SNP supporters who are particularly likely to go and vote for Greens on the second vote.”

Dr Craig McAngus, a politics lecturer at the University of Aberdeen, who has reviewed the data, said: “If you look at the numbers and how things stack up in certain areas - such as Glasgow where I’d imagine every single constituency seat would go to the SNP - there is an argument that if you are pro-independence and left-leaning then why not bank on one of the other alternative left-leaning pro-independence parties?”

He added: “To vote for another party other than the SNP on the list as a pro-independence voter is a rational thing to do. I mean, the way that the numbers stack up, the SNP are not going to win a lot of seats on the list.”

Mark Diffley, director of Ipsos MORI Scotland, described Curtice’s report as “sound”, adding: “He’s crunched the numbers and it’s pretty self-evident to people who understand the voting system. There’s nothing magic in his numbers.

“He’s basically made some sensible assumptions about swing, and crunched them through the system as we know it works in terms of the regional vote and come up with some numbers showing the SNP will only get two regional seats, both in the Highlands and Islands.

“What he’s suggesting is that because of the strength of the SNP’s constituency vote and the number of seats they’ll pick up there, which is virtually all of them, there’s a greater chance if you vote SNP in the second vote – depending on which region you’re in – that it won’t result in the SNP winning a regional seat.

“The assumptions he makes to get to his conclusions are pretty clear. It’s basically a way of explaining how the electoral system works and a lot of people don’t really understand that.”

The report’s conclusions were welcomed by the Greens and Rise – two parties named by Curtice as a regional vote option for backers of Scottish independence.

Rise candidate Cat Boyd said: “In this election, we're asking people to vote for what they believe in. I'm not campaigning for any other electoral force than the one I'm standing for, I think it would be unprincipled to do otherwise. All I can say is: tactically, use your first vote for the government you want; morally, use your list vote for the values you want in politics.”

Co-convenor of the Scottish Greens, Patrick Harvie MSP, said pro-independence supporters who follow the ‘Both votes SNP’ directive - which has become a hashtag online - “will be cutting off their nose to spite their face”.

He added: “If the question of independence is put again, it will need a broad platform with non-SNP voices such as the Greens.”

Paul Cairney, professor of politics and public policy at the University of Stirling, suggested that the SNP would prefer to be the only pro-independence party at Holyrood.

He said: “The assumption is people vote based on their constitutional position. Therefore if you’re not going to vote SNP you vote Greens or Rise if you support independence.

“That’s a tricky one because the Greens and Rise will be the biggest critics of the SNP by far because the SNP are closer to Labour and the Conservatives on lots of issues.

“If you compare the manifestos of the SNP and Greens, that’s the biggest contrast. I think the SNP would rather have more Conservatives in there because the Conservatives will push for tax reduction and then the SNP can look socially democratic by comparison.”

However, SNP MP Mhairi Black said leader Nicola Sturgeon will use Holyrood’s new tax powers to raise more revenue for public services.

She also took on the split vote issue for independence supporters aggressively, saying: “Those who back independence should – so goes the line - just take a punt and give that second vote to a party other than the SNP which claims to support independence, thus, supposedly, maximising the pro-independence strength in parliament. All of that is wrong, misguided and – in some cases – profoundly disingenuous.

“If people want an SNP Government to be re-elected, Nicola Sturgeon to win her own mandate as First Minister for the first time and be in a position to take forward and implement this week’s manifesto then the only way – the only way – to be sure of delivering that result is to give both votes to the SNP.”

Scottish Labour and the Conservatives are currently battling it out for second place, according to at least one recent poll, but in the ERS report Curtice projected Labour will end up with 25 seats while the Conservatives will win 18.

Meanwhile, public affairs firm Weber Shandwick has averaged the results of six opinion polls conducted during April, and the SNP are set to take 72 seats (+3), Scottish Labour 23 (-14) the Conservatives 19 (+4), the Greens 9 (+7) and the Liberal Democrats 6 (+1).

A Scottish Labour spokesman said: “We are advising our supporters to give both votes to Labour. We are asking them to do this because they support the Labour Party.”

A spokesman for the Scottish Conservatives said: “Our advice is quite simple: if you believe in a successful Scotland within the UK and want a strong opposition to the SNP, both votes should be cast for the Scottish Conservatives.”

The Sunday Herald also contacted the Scottish Liberal Democrats but they did not respond to a request for comment.