TACTICAL voting for the Greens by SNP supporters in 2016 increased the number of pro-independence MSPs rather than boosting Unionist numbers at Holyrood, the first major report on last year's election has found.

Last night the Greens said independence supporting Scots had "saved the chance of a referendum in this parliament" by splitting their votes tactically.

The Getting to Minority Government report from the Electoral Reform Society (ERS) Scotland, written by one of the UK’s top political scientists, Professor John Curtice, will be published this week.

During last year's election campaign, there was controversy over claims that Yes supporters should consider giving their second vote to another pro-independence party – such as the Scottish Greens - in order to keep unionist parties out of Holyrood.

Nicola Sturgeon ran a high profile Both Votes SNP campaign, with a warning to her supporters that voting for pro-independence alternatives on the regional list could let Unionist MSPs in via the back door.

However, the claim has now been dismissed in the first significant report on voting behaviour in the 2016 Scottish Parliament elections.

The election saw pro-independence parties at Holyrood win a majority, when the SNP and Green MSPs are combined, even though Sturgeon's party narrowly lost the overall majority it had in the previous parliament.

“Voting Green in the list vote tended to win Greens seats rather than allow pro-Union parties wins", according to Willie Sullivan, Director of ERS Scotland, in a foreword to the report.

In his report, Curtice, highlighted the shift in SNP supporters voting tactically for the Greens on the list to boost the number of pro-independence MSPs, but still backing Sturgeon's party in the constituencies.

Curtice, a professor of politics at the University of Strathclyde, said that if there had not been such tactical voting there would have been fewer pro-independence MSPs.

The report said the loss of the overall majority the SNP had in the last parliament was mainly due to its failure to take enough target constituency seats rather than because of the gains made by the Greens on the regional list.

Curtice said: "It is evident that the apparent tactical switching in favour of the Greens had less impact on the SNP’s overall tally than the party’s failure to win seven constituency seats that it might have been expected to have won."

He found that tactical voting by SNP supporters for the Greens explained the rise in the number of the party's MSPs from two to six.

The Scottish Greens share of the vote on the regional list last year went up by 2.2 per cent on that from the 2011 election.

Meanwhile, the SNP won 63 seats – two short of a majority and with its share of the list vote going down by 2.3 per cent.

Without this shift the Greens would have won four fewer seats, leaving them on the two seats that they won in 2011, according to Curtice.

However, only two of those lost seats would have been claimed by the SNP, with the other two going to Labour or the Conservatives, he said.

Curtice said: "Two extra seats would have been just enough to deliver the SNP a majority with 65 seats. At the same time, however, there would have been two fewer MSPs in favour of Scottish independence."

He highlighted the shift in voter behaviour during the last parliament, which included the independence referendum campaign.

"It looks as though voters who voted for the SNP on the constituency ballot were more inclined than they had been five years previously to vote for the Greens on the list vote, most likely because they thought that this would be the best way of maximising the number of MSPs in favour of independence.

"All in all, there is considerable evidence that SNP voters were more likely to vote tactically for the Greens on the list ballot in 2016 than they had been in 2011", the authors of the report found.

Green West of Scotland MSP, Ross Greer, welcoming the findings, said: “This report makes for some interesting reading and does confirm that those who voted Green/SNP to maximise the number of pro-independence MSPs made the right tactical decision and possibly saved the chance of a referendum in this parliament."

An SNP spokesperson, in response, said "Last summer's election provided a strong mandate for the SNP and an unprecedented third term in government."