THE failure to get an SNP majority at the Holyrood election could be dismissed by Boris Johnson as not securing a mandate for a second independence referendum even if the party wins the most seats, pollsters have warned.

Sir John Curtice, professor of politics at Strathclyde University, who is also the president of the British Polling Council, said forming a pro-independence coalition government with the Scottish Greens to seek a Section 30 from the UK Government to hold a new vote would be unlikely to have the same political power as an SNP ­majority government.

He was among the experts who gave their views to The National after Patrick Harvie, the Scottish Greens co-leader, said he was open to forming a coalition government with the SNP.

Harvie’s intervention came amid concerns the continuing row between First Minister Nicola Sturgeon and her predecessor Alex Salmond could have a negative impact on the SNP’s chances of winning a majority at ­Holyrood in the May election.

“A coalition is irrelevant. I think it would be easier for the Unionists to deny the SNP a referendum if the SNP don’t get an overall majority,” he said.

“Just compare and contrast. In 2011 the SNP won a majority and David Cameron agreed to the referendum, in 2016 there was a pro-independence majority at Holyrood, but in 2017 Theresa May says no.”

He added that the advantage for the SNP in having a majority is that the party can cite the agreement the Scottish Government got from Cameron and ask why such an agreement cannot be repeated by Johnson.

Curtice’s assessment was also shared by Chris Hopkins, associate director of polling company Savanta ComRes.

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Asked what impact a SNP minority government or a pro-independence coalition with the Greens would have on seeking agreement with Johnson over a second independence referendum, he said: “I think both would weaken their case for a second ­referendum.

“Ultimately the SNP, not the Greens, are synonymous with a ­desire for independence, and if the SNP fail to achieve a majority, it would strengthen the Unionist argument that there isn’t a strong enough mandate for a referendum, no matter what Patrick Harvie might say about Holyrood still being majority pro-independence.”

Hopkins went on to say that Harvie being open to a coalition made some sense – though he doubted whether it was a realistic prospect.

“Its likelihood is a different story,” he said.

“Had you asked me a few weeks ago I’d have said the chances that the SNP would need a coalition would be slim. They looked well on course for more than 50% of the constituency vote which would likely be enough for a majority without even needing much of a top-up from the list.

“However, the harassment inquiry does throw the cat among the pigeons a bit, and although we don’t know at this stage what, if any, electoral impact it may have, if it does have a negative impact then yes, the SNP could be reliant on the Greens to form a majority.

“And while the green parties across Europe do tend to fare OK in ­coalitions, British politics is inherently less used to them and welcoming of them – one only has to look at how the Liberal Democrats have fared.”

He added: “There’s also nothing to say the SNP would not choose to govern as a minority government again, as they did in 2016. This statement from Harvie may be something of a plea to the SNP to consider them and achieve a majority if they need it, but precedent doesn’t necessarily mean the Greens will end up in government come May.”

A Scottish Greens spokesman said: “This is a simple case of ­arithmetic, usually one of Sir John’s strong points. If a majority of MSPs in the next ­Scottish Parliament want a ­referendum on independence, then the case for one will be unanswerable. The Scottish Greens already provide the Parliament with such a majority, and polls suggest we will do so with even ­greater numbers after May.”