BORIS Johnson will be forced to cave in on staging indyref2, Ian Blackford has insisted, claiming that UK Cabinet ministers have privately conceded the Prime Minister’s opposition to a second poll is unsustainable.

The SNP’s leader at Westminster even suggested Mr Johnson himself knew, in his heart of hearts, that his iron resistance to facilitating a second poll on Scotland’s future would ultimately be broken.

“This dam will burst,” he declared.

In an exclusive interview with The Herald at the beginning of a year in which the constitutional debate will be to the fore, Mr Blackford accepted that, with a Tory Government bolstered by an 80-seat majority, the SNP would have to “up our game”.

READ MORE: Scottish Secretary rules out Indyref2 even if SNP win Holyrood majority in 2021 

This, he suggested, was the reasoning behind his reshuffle of his Commons frontline team last week to place the focus on “engagement…with external organisations whether that’s at home or abroad,” indicating the SNP is set to try and garner support for its independence cause, including indyref2, from allies across the EU.

“We will make sure we are building the case both for Scotland to be independent and for the right to have that independence referendum at home and abroad. So, we are simply not going to let this go,” declared Mr Blackford.

His comments come after Scottish Secretary Alister Jack yesterday said Mr Johnson would block Indyref2 even if the SNP won an outright majority at next year’s Holyrood election.

He told BBC Scotland’s Sunday Politics show: “We do not think that 2021 is the time to start having more referendums. They’re very divisive for our society.”

It was a U-turn on his position in November, when Mr Jack said another SNP majority would, as in 2011, constitute a “democratic mandate” for a referendum.

Instead, Mr Jack said one of Ms Sturgeon’s future successors would have to wait until a generation or a lifetime had passed before the No result of 2014 was revisited.

Mr Blackford's insistence that the PM will be forced to give way on indyref2 comes after tens of thousands of people took part in a pro-independence protest march in Glasgow at the weekend – the first of eight marches being planned by the group All Under One Banner.

Keith Brown, the Depute SNP leader, who took part in Saturday’s protest, said: “If the UK remains a democracy, then Scotland must have the right to choose. Plain and simple.”

Rebecca Long-Bailey, tipped by some as the favourite candidate for the Labour leadership, told Sky News’s Sophy Ridge on Sunday that she was “fully committed to the Union” and was not in favour of a second independence referendum but suggested, if she became PM, she would “review” indyref2, noting: “I wouldn't want to inhibit the democracy of people.”

Scottish Labour leader Richard Leonard suffered a setback after the party’s executive on Saturday rejected his proposal for a special conference on federalism, which could have resulted in the party backing a multi-option referendum on independence.

READ MORE: Mark Smith: If you can’t beat ’em, join ’em. What I learned from my first independence march 

Last week at another acrimonious PMQs, Mr Johnson once again made clear that he had no intention whatsoever of countenancing a second referendum in 2020 or indeed in this five-year parliament while Mr Jack pushed back hard on calls for Holyrood to have power on referendums, saying this would be “completely wrong” as it would lead to a “series of neverendums”.

But Mr Blackford asked: “How many times do the people of Scotland have to vote for the SNP to give the Scottish Parliament a mandate to have an independence referendum? Westminster cannot keep saying no.”

He went on: “Is he really prepared to ignore a party that has got 80 per cent of the seats from Scotland in this place and that has won 45 per cent of the vote? There is no Government over the last couple of decades that has won with a share of the popular vote above 45 per cent since 1966. It’s extraordinary.”

When it was suggested the answer to his question was yes – the PM has made clear time and time again that Scots were asked a once-in-a-generation question on their future in 2014 and opted to stay with the Union – the Highland MP replied: “Events are going to determine this.

“I have had conversations with a number of Conservative ministers privately and they understand that this is a line which is going to be difficult to hold for the longer term.”

Pressed if he was talking about Cabinet ministers, he replied: “Yes. I’m not going to name them. But I don’t believe Boris Johnson sees that any differently to some of the other people I’m talking about.”

Asked if he was seriously suggesting the PM was simply grandstanding and that he believed in his heart his opposition to indyref2 would not hold, Mr Blackford said: “I believe there are a number of Conservatives in the Cabinet who recognise that position.”

During PMQs, the SNP leader was barracked by Conservative MPs when he based his argument for indyref2 on democracy (ie his party had won the vote in the election in Scotland so, ipso facto, another referendum should follow.)

But it was pointed out that the election was a UK one and Mr Johnson won an 80-seat majority premised on a manifesto that pledged not to facilitate a second independence referendum. Surely, Mr Blackford was asked, in a democracy a majority vote ruled.

“No, that’s not democracy. That’s a wrong reading of the situation. Because what you are actually saying is you can vote for the SNP and you can vote for the right to call a referendum under the Scottish Government and Westminster is always going to ignore that. That is not right because that impinges on the right of self-determination that countries have.”

When Mr Johnson’s argument was repeated – that Scottish voters had an opportunity to vote for independence five years ago and rejected it – the party leader insisted: “You cannot take the view that democracy finished when the polls shut on September 18 2018. That’s an absurd argument.”

When it was suggested his argument effectively negated any vote, the party leader bristled and pointed out Westminster had backed the Claim of Right in 2016, which upheld the sovereignty of the people of Scotland to choose their own future.

From his oak-panelled Commons office, the MP for Ross, Skye and Lochaber, who at the election increased his majority by almost 60 per cent, accepted Westminster had now, post the poll premised largely on Brexit, moved on.

“We are moving into a different phase. The phase of this being a minority government, everybody being here every day, lots of late nights, lots of votes[has gone]; the Government will behave in a different way and we will behave in a different way.”

Mr Blackford argued Mr Johnson’s obstinacy would “push people into the hands of the SNP”.

He denounced as “utterly disrespectful” the failure so far for the PM to reply to the First Minister’s letter of December 19 about indyref2; a fulsome response is expected soon, possibly this week.

When it was suggested there was absolutely no way the PM, after all that had happened in the last three years over Brexit, would facilitate what would be another deeply divisive referendum with the possibility of protracted talks on taking Scotland out of the Union, the Nationalist leader said: “I simply say to Boris Johnson: carry on. Because this will be to your cost.

“They will not be able to defend this position. I really do believe we can move the needle on this.”

“If I’m wrong, then this plays out in the Scottish election in 2021; not my choice because I want this resolved before then.

“But if the Government does not give way on this, then this becomes the backdrop to the 2021 election. Now do the Conservatives in London really want the next Scottish election want to be about the right of Scotland to choose its own future?”

Mr Blackford refused to rule out future court action, saying the SNP would take the process “step by step” and that he was “not going to declare our hand in terms of the options we have”.

But he added: “We need to up our game, we need to demonstrate that public support for independence is there. The obligation is on us to do that...

“We will accept that challenge; we will campaign door by door, street by street, township by township, village by village, we will build support for independence. This dam will burst.”