GROWING splits in the SNP are the “biggest risk” to the party achieving independence, the UK’s leading polling expert has warned.

Professor Sir John Curtice said one of the Nationalist side’s great advantages over its Unionist opponents was that it had been “effectively organised by one party”.

That contrasted with the “fragmented” nature of the No campaign in 2014, which he said was a “fundamental weakness”.

However the emergence of new Yes parties put SNP unity at risk.

The Independence for Scotland and Alliance for Independence (AFI) parties have registered with the Electoral Commission in readiness for the Holyrood election.

The AFI, led by former SNP MSP Dave Thompson, has announced it will stand in all eight regional lists in a bid to mop up Nationalist votes it claims would be “wasted” on the SNP because of the way the electoral system works.

But the SNP has refused to withdraw from the lists, meaning the Yes vote could split.

The Bath-based blogger Stuart Campbell, who runs Wings Over Scotland, has also said he is considering forming his own party.

While Solidarity leader Tommy Sheridan has urged Alex Salmond to “sweep up” list votes by leading his own pro-independence party.

The developments have been driven by frustration in the Yes movement over slow progress towards a second referendum, with Boris Johnson blocking a new vote.

Mr Salmond’s personal rift with Ms Sturgeon is another factor.

Prof Curtice told the Courier newspaper: “The brutal truth, it seems to me, is that the biggest risk the SNP face to the realisation of their ambitions are their own internal divisions.

“Obviously Nicola Sturgeon will face the difficulties of the handling of the Alex Salmond affair and there is a reasonable debate to be had about what are you going to do if you get an overall majority and the UK Government does still say no.

“I suspect Sturgeon will be wise at some point to say a little bit more about that but she’s obviously determined at the moment to say, ‘I’m just not talking about independence at all’.”

Despite the AFI saying its MSPs would be “bound” to keep a pro-independence government in power, Prof Curtice said Ms Sturgeon would not want to be “in hock” to another party.

He said anything short of an outright SNP majority would weaken her demand for Indyref2.

“At the moment, the opinion polls are saying you’ve never had to worry less about the need to game the system,” he said.

“For the moment, at least, the SNP are heading for a whopping great majority, anyway.

“Things may be different next May but right now one would say it’s just not necessary.

“The obvious risk, then, is what the unionist side is going to make of it if there is a significant and serious list vote, particularly with the idea of putting Alex Salmond at the head of the list.”

“What you want to do, from a nationalist perspective, is maximise the chances of there being an SNP-only majority in Holyrood after next May.

“I’m assuming the SNP will say something much less circumstantial than in 2016. For example, we want a referendum full stop within the next parliament, not if circumstances change.”

“If they make the same promises as in 2011 and they get the same outcome as 2011, the UK Government will be absolutely on the back foot.”

Prof Curtice said such an outcome would be the hardest for Mr Johnson to resist, despite his current insistence that the No result of 2014 must stand for a generation.

“Sure, you can keep on arguing that the SNP said once in a generation and that they should keep their promises but what you cannot do is bind the electorate.

“And if the electorate decide that they wish to back another referendum and they give the SNP an overall majority, then you cannot say we are going to ignore you.

“The electorate cannot be bound by the promises that were made by politicians in 2014. The electorate have the right to be fickle, even if your opponents don’t.”

He said if the PM dug in, the issue could go to the UK Supreme Court, and Ms Sturgeon ought to indicate what her Plan B would be if she failed to get her way.

“I think she would be wise, partly just to keep her troops happy, just to indicate what her alternative would be.”