HOLYROOD can “grind down” the Tory Government with "successive constitutional disputes" if it continues to refuse Scotland an independence referendum, an SNP MP has said.

Pete Wishart, his party’s longest-serving member at Westminster, said Scotland could heap pressure on London into granting a new vote by making the consequences of denying one so difficult.

Writing on his blog, he said these could include withdrawing from “inter-governmental machinery and institutions”. 

"MPs should continue to serve our constituents and speak out in Parliament but it would not be business as usual," he wrote. "In the Scottish Parliament legislative consent could be increasingly withdrawn. 

“The Scottish Parliament could also start to legislate on issues at the edge of the devolved powers challenging the UK Government to strike Scottish democracy down again. 

“We could then start to grind them down with successive constitutional disputes.”

With a series of polls pointing to most Scots now backing independence – with one poll putting the figure at 58% – and that the SNP will win a majority at next May's Holyrood election, Wishart said he believed Boris Johnson's government would agree to an early ballot.

However, the MP for Perth and North Perthshire added that Scotland could “start to put in place” its own referendum if the UK denied Holyrood the powers for an agreed one. 

He stopped short of saying Scotland should actually hold such a referendum – though he argued the move would be used to rally international support and shame the UK into cooperation.

He said: “The UK will agree to a referendum if we win in May. They will have looked at the consequences of saying No just as we have. They will not like the look of it at all.” 

Prime Minister Boris Johnson has refused to grant Holyrood the Section 30 power needed to stage a legally watertight referendum, arguing the No vote of 2014 was a once in a generation event

Scottish Secretary Alister Jack has also indicated another vote should not happen for up to 40 years.

Some in the SNP, including MP Angus MacNeil and Inverclyde councillor Chris McEleny, have demanded a “Plan B” to overcome such resistance from London, arguing an SNP election win should be regarded as a mandate for independence.

However, the First Minister has said the referendum of 2014 remains the gold standard, as it would the result would legally beyond doubt and internationally recognised.

Wishart, shadow minister for the Cabinet Office, has also challenged those arguing for a Plan B, and called for patience and resolve.

The National: Pete Wishart will chair the committee hearing the evidence. Photograph: Getty

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He said a “strange gloom and fatalism has taken hold” in parts of the Yes movement, with people convinced the UK Government will never agree to another plebiscite.

He said: “I am sincerely of the opinion that the Tories will agree to another referendum particularly if the conditions of 2011 are recreated and there is another SNP majority. 

“This, combined with a majority in favour of independence means the only chance the Tories have of saving their union will be to agree to an early contest. 

“They know that to continue to say ‘No’ they will only drive support for independence even higher.”

He said that if the UK continued to say No, a Unilateral Declaration of Independence would “appall” the EU, which an independent Scotland would want to rejoin.

It would also be a “huge ask” for the Scottish people to accept it, as it would be a break with the SNP’s tradition of democratic campaigning.

Instead, he said that if London still said No, Scotland should “turn up the heat, start to withdraw consent and build up international capacity”, while giving the UK Government “every opportunity to engage”.

He said: “We can allow them to draw up any agenda they want (as long as a referendum is on it) and we say to them we are prepared to meet any day, anytime, anywhere. 

“We have to demonstrate to the international community and the EU that we have done everything possible to get them to the table. We can allow them to draw up any agenda they want (as long as a referendum is on it) and we say to them we are prepared to meet any day, anytime, anywhere.

"We have to demonstrate to the international community and the EU that we have done everything possible to get them to the table. At the same time, if a No persists we should be starting to think about withdrawing from the structures of the UK state.” 

He said the choice would be either a straight forward process that respected democracy or “a messy, drawn out affair” that could sour long-term relations.

He said: “All of this will be the UK’s choice. What we must not be drawn into is precipitative action that would put us on the wrong side of the international community. 

“There will be strong lobbying from some within the movement to embark upon actions that would lead to that situation, just as there will be provocation from the UK to divide us and try and force us down a route unpalatable to the international community and the Scottish people. Patience but determination will be required.

“I sincerely believe that the UK will not want any of this just as much as no one in our movement wants it either. 

“That is why I come back to where I started – the UK will agree to a referendum if we win in May. They will have looked at the consequences of saying No just as we have. 

“They will not like the look of it at all. 

“But it will be all down to them. Whatever they choose – we win, because we must.”