SUPPORTERS of independence should not give their second vote to the SNP at the Holyrood elections, according to a new report by one of the country's top political scientists.

Professor John Curtice says Yes voters should instead give their backing to another pro-independence party - such as the Greens or the leftwing party Rise - in case unionist MSPs are let in by the back door.

The controversial claims are a blow to the 'Both Votes SNP' campaign. The SNP responded to the Curtice study saying voters who followed his advice "risk playing into the hands of those who oppose a fully self-governing Scotland".

Curtice’s report also predicts that Labour will remain the official opposition with a reduced number of seats, crushing boasts by the Conservatives that they will come second in the May 5 ballot.

In the study commissioned by the Electoral Reform Society - which campaigns for greater democracy - Curtice collated and analysed recent opinion polls and found that the SNP will win all but three constituencies and be returned firmly as the majority party of government on that vote alone.

Due to the rules of Scotland's complicated electoral system that would mean the SNP would then see only two MSPs returned on the regional lists – most likely in the Highlands and Islands because they will win fewer constituencies in that region.

Pro-independence voters in other parts of Scotland who cast both votes for the SNP could therefore be “wasting” their regional list vote, according to Curtice.

Curtice underlines the fact that any party which hopes to win a regional seat must secure around 6% of the vote, suggesting that voting SNP in the regional ballot instead of smaller pro-independence parties could benefit unionist parties with a bigger share of the vote and no constituency MSPs.

He said: “This year’s election is being fought under a system of proportional representation that, although it may well give the nationalists another overall majority, will also ensure that any party that wins more than 5-6% of the vote will secure at least some representation.”

Co-convenor of the Scottish Green, Patrick Harvie MSP, said: “I’ve already spoken to a number of voters who feel a bit taken for granted by the ‘both votes SNP’ slogan, but it seems clear that even those who do decide to follow that strategy will be cutting off their nose to spite their face...If the question of independence is put again, it will need a broad platform with non-SNP voices such as the Greens.”

Prominent Rise candidate Cat Boyd said: “John Curtice has made the case that we need to vote with our heads as well as our hearts. I'd rather people voted for Rise on the second ballot than let unionist parties in by the back door.”

Curtice has also poured cold water on suggestions that the Conservatives could win more seats than Labour and usurp them as the official opposition.

Average polling puts Labour at 20% (down from 32% in 2011) suggesting the party could end up with only 25 regional list seats because it is “more or less confined to fishing for votes in the waters of unionist voters and finding itself in competition with the Conservatives in so doing,” according to Curtice.

But the academic also points to average polling data which shows support for the Conservatives in Scotland is at 15% - up only 1% since 2011 and still 2% down on the figure the party achieved in 2007. Seat projection suggests leader Ruth Davidson could therefore end up with 16 list MSPs and two constituency MSPs, far fewer than she would need to lead the opposition.

Curtice added: “In short, the speculation about second place is more an indication of Labour’s difficulties than of Conservative progress.”

The University of Strathclyde professor also predicts in his report that the Scottish Green Party and the Scottish Liberal Democrats will gain seats while other parties will not make a breakthrough. The number of Green MSPs is expected to rise from two to eight while the Lib Dems will gain one MSP taking their tally to six.

Director of Electoral Reform Society Scotland, Willie Sullivan, said the report is intended as a resource for voters who want to understand the election - but the analysis clearly has implications for the smaller parties whose support could be squeezed by the ‘both votes SNP’ strategy.

“We think politics should contain lots of different voices,” said Sullivan. “In the past we have stated our concern about the predominance of a single party in Scottish politics.

“This was true when it was Labour under the Westminster system and is true now with SNP dominant at Westminster under First Past the Post and looking to become even more so in Holyrood.”

A spokesman for the SNP said: “Only by giving both votes to the SNP can people be sure of returning Nicola Sturgeon as First Minister, leading a re-elected SNP Government in a position to take forward our manifesto plans.

“While we welcome other parties' backing for independence, fracturing the pro-independence vote merely risks playing into the hands of those who oppose a fully self-governing Scotland.”

Read the full report here: