THE SNP should not reveal a Plan B before the May elections, according to polling expert Sir John Curtice.

However, he also believes the ­current internal fighting is a risk to the party’s strong position in the polls.

While he thinks the party will continue to benefit from the “Brexit ­effect”, at least until May, the boost to the ratings that has resulted from the Scottish Government’s ­handling of the coronavirus pandemic will depend on what happens over the next “crucial” months.

And as the Holyrood elections draw nearer, any infighting within the SNP may lose the party a lot of support.

READ MORE: Opposing Brexit deal will help SNP seal majority, John Curtice says

Political pollster Mark Diffley also believes ­voters who have switched to the SNP because of their opposition to leaving the EU are likely to stay with the party at least until after the ­Scottish Parliament elections.

“What will play into the SNP hands is the fishing issue as the concessions the UK Government has made in the Brexit deal will not have pleased the Conservatives in Scotland one bit,” he said.

Diffley said he didn’t think the SNP’s decision to vote against the agreement last week would make any difference to their standing in the polls and the fact that Labour had supported it would probably backfire in Scotland.

He added that support for the SNP is strong not just because of Brexit but because the Scottish Government are perceived to have handled the coronavirus pandemic far better than Westminster.

READ MORE Independence: Plan B would take pressure off PM, John Curtice says

Curtice said there are “two ­pillars and one risk” to the SNP’s ­current ­position, with Brexit and the ­handling of the pandemic forming the two ­pillars and party infighting comprising the risk.

While the party would be able to continue to capitalise on anti-Brexit feeling in Scotland he said the coronavirus boost would depend on what happens next.

“We are clearly entering the most difficult period,” warned Curtice, ­professor of politics at Strathclyde University. “Scotland is in a somewhat better position than the rest of the UK at the moment but this will be a tough few weeks.”

He pointed out that the Welsh Government’s support had “plummeted” recently as the virus had taken hold.

The main risk for the SNP, however, is the current infighting, according to Curtice. He said although recent polling showed voters were not paying much attention to it, that could change.

“There clearly is a debate to be had there about tactics. I suspect that if they were to sit down, [Nicola] Sturgeon and [Joanna] Cherry would discover they largely agree – it is just a question of what you say in public as ­opposed to what you say in private,” said Curtice.

The National:

“But one of the ironies is that at a time when the SNP’s position has never looked stronger and independence has never looked more popular it seems to be the moment when internal strife is more serious than at any time since the late 70s and early 80s. The things I would be worrying about are coronavirus and keeping my ­party from doing an injury to ­itself because I suspect the benefits of Brexit are unlikely to unravel.”

Curtice said that while the SNP should have a Plan B on what to do if Westminster refuses to agree to another independence referendum, it should not be revealed before May.

“The point is that you don’t want to say it this side of May because if you are going to maximise the pressure on [Boris] Johnson you have to say ‘we want a referendum and voting for us is a mandate for a referendum just like the last one’.

READ MORE: John Curtice: SNP MPs could 'gum up' Westminster until Tories agree to indyref2

“If you say what your Plan B is the UK Government can then say ‘go and do it, we don’t like Plan A’.

“You don’t want to declare it and equally of course after May, if the SNP get their overall majority, the UK Government will have to think about the best way of playing it and simply saying ‘no’ may not be the best way. It is in the interests of both sides to be economical with the truth between now and May but certainly both sides need to have a Plan B.”

Curtice also cast doubt on the wisdom of the crowdfunded move to take the referendum issue to the Court of Session on the grounds that knowing whether the SNP has the power to hold a referendum without Westminster’s consent would not do the party any good before the elections.

“I take the view that the strategy the SNP should be pursuing between now and May has to be ‘we want a referendum just like in September 2014’ full stop. In exactly the same way the stance of the Conservatives has to be ‘we think a referendum is a damn ­stupid idea’,” he said.