Blackford fails to say if an imminent snap election would be 'de facto' independence vote

However, he could not say whether any imminent vote would be the proxy referendum on the country's constitutional future.

Under the First Minister's route map unveiled last week she announced plans to hold Indyref2 on 19 October 2023 if the Supreme Court found her proposed bill using devolved powers lawful.

She said if the legislation was not found to be lawful the SNP would then campaign at the next general election - then expected in 2024 - on the single issue of whether Scotland should be an independent country and that this event would be a "de facto referendum".

However, it is now possible that with Johnson's government on the point of collapse following a string of senior ministers a new Tory leader may call a snap election.

Asked if a snap election taking place soon would be a referendum on an independence referendum, Mr Blackford said: "What we said first and foremost is that we want to take this to the Supreme Court, let's do that, and set a course.

"Let's assume that we win that referendum but if we don't then we will look at our tactics for an independence referendum using a general election.

"But the sequence of that will be going to the Supreme Court first and if there is an election before that we of course will think long and hard how we fight that election.

"But it will be about Scotland's right to have a referendum to be an independent country."

The SNP’s leader at Westminster repeated his demand for the Prime Minister to go and later repeated the plea at Prime Minister's Questions where he urged the PM to call a general election if he refused to resign.

"Whether he knows it or not, he's now an ex Prime Minister," said Mr Blackford.

"He will leave behind two deeply damaging legacies. I hope the dishonesty of his leadership follows him out of the Downing Street door.

"But the other legacy is Brexit and that will stay because I'm sad to say the Labour party now fully supports it.

He added: "Scotland wants a different future not just a different Prime Minister. So if the Prime Minister won't resign will he call a general election and allow Scotland the choice of an independent future?"

Responding to Mr Blackford, the PM insisted that like the SNP, Labour wanted to return to the EU.

He added: "As for the referendum he wants, we had one of them...in 2014."

Mr Blackford said Mr Johnson has lost the trust of both voters and his party and should step aside to allow Westminster to focus on the “immense challenges” facing the country.

Mr Blackford was speaking after Will Quince resigned as Mr Johnson’s children and families minister on Wednesday, and Laura Trott quit as a ministerial aide. She said trust in politics “has been lost”.

Their resignations followed a string of departures from Mr Johnson’s Government on Tuesday evening, led by chancellor Rishi Sunak and health secretary Sajid Javid.

Mr Blackford told Sky News the number of resignations has reached “crisis” point.

He said: “One man now needs to accept he is the problem, that he has lost the trust of the people of these islands, that he has lost the trust of the House of Commons and indeed a vast number of his MPs.

“He now needs to realise he needs to go.

“We’ve got Prime Minister’s Questions today and there’s lots of things we could be talking about, we need to be talking about the cost-of-living crisis, the war in Ukraine, but we won’t be because we will be speaking about Boris Johnson.

“He’s now a block on us in the House of Commons doing the job we need to do, and for the good of everybody Boris Johnson has to go.

“He simply shouldn’t be here. We stumble on from crisis to crisis. There has to be an end to this.

“We need to be able to go on and tackle the immense challenges we have.”

Mr Blackford described the Prime Minister as “a man who has no integrity, a man who has no shame”.

He added: “For the sake of everyone and for his own dignity and self-respect, he needs to realise this is over.

“We’re going to go into the winter very soon, the cost-of-living crisis is very much with us, we need to make sure we are dealing with inflation.

“We need to make sure we are dealing with the fundamentals and we can only do that when we lance the boil that is there, when this Prime Minister is gone from office.”

His comments came as a former Scotland Office minister told BBC Radio Scotland that more resignations from the Government could make Mr Johnson’s position “untenable”.

When asked if Britons are now witnessing the beginning of the end of the Prime Minister’s time in office, Lord Duncan told the Good Morning Scotland programme: “Yes, I think we are now.

“How quickly it unravels will depend on factors out-with the Prime Minister’s control, but yes we are.”

Meanwhile, Mr Johnson has insisted he will not leave No 10 despite a mounting revolt against his leadership.

Ministers and aides have continued to submit their resignations, while support is ebbing away from the Prime Minister among previously-loyal MPs.

But Mr Johnson is understood to have told allies that he is "not going anywhere" and his critics should "calm down".

On Wednesday morning, Robin Walker resigned as schools standards minister, telling the Prime Minister the "great achievements" of the Government have become "overshadowed by mistakes and questions about integrity".

Their resignations followed a string of departures from the Government on Tuesday evening, led by Rishi Sunak and Sajid Javid, who delivered broadsides at Mr Johnson as they quit their Cabinet posts.

Former health secretary Mr Javid is expected to add to Mr Johnson's problems with a personal statement in the Commons on Wednesday.

This will come after the Conservative leader faces a potentially difficult session of Prime Minister's Questions, while he will also have to endure a grilling by the Liaison Committee of select committee chairmen and women - including some senior Tory critics.

Education Select Committee chairman Rob Halfon, one of those who will question the Prime Minister, said he would back a change in leadership, criticising not only a "real loss of integrity" but also "a failure of policy".

But Mr Johnson told friends he will continue to "smash on and deliver for the people who gave us a massive mandate", the Daily Mail reported.

"Everyone just needs to calm down, stop bickering and let us get on with the job in hand."

New Chancellor Nadhim Zahawi hinted at reversing a planned rise in corporation tax as part of the effort to win over Tory MPs.

But the Cabinet reshuffle does not appear to have persuaded Mr Johnson's critics to hold fire.

Mr Quince was one of the ministers sent on the airwaves to defend Mr Johnson's position over Chris Pincher, who quit as deputy chief whip after allegedly assaulting two men while drunk at London's Carlton Club.

The Prime Minister later acknowledged he had previously been informed of allegations against Mr Pincher dating back to 2019 and said he regretted keeping him in government beyond that point.

Mr Quince said he had received a "sincere apology" from Mr Johnson for being sent out with an "inaccurate" briefing about the Prime Minister's knowledge of events.

But "I have no choice but to tender my resignation" as "I accepted and repeated those assurances in good faith".

Mr Walker's resignation letter to the Prime Minister said: "Recent events have made it clear to me that our great party, for which I have campaigned all of my adult life, has become distracted from its core missions by a relentless focus on questions over leadership."

He added: "I have always believed it is the job of our party to strike the right balance between efficiency and compassion, but the image being projected from the struggles of the last few months is that we risk achieving neither."

The Prime Minister's authority had already been damaged by a confidence vote which saw 41% of his own MPs withdraw their support in June.

The loss of crunch by-elections in Wakefield and Tiverton and Honiton later that month triggered the resignation of party chairman Oliver Dowden, while there is still lingering anger over coronavirus lockdown-busting parties in Downing Street.

Tory MPs are also uneasy about the Government's high-spending, high-taxing approach as a result of the response to the pandemic.

Mr Zahawi sought to reassure Conservatives that "nothing is off the table" when questioned about possibly scrapping the planned increase in corporation tax from 19% to 25% in April 2023.

"I will look at everything. There's nothing off the table. I want to be one of the most competitive countries in the world for investment," he told Sky News.

"I know that boards around the world, when they make investment decisions, they're long term, and the one tax they can compare globally is corporation tax. I want to make sure that we are as competitive as we can be whilst maintaining fiscal discipline."

The Prime Minister's fate may ultimately lie with backbench MPs if the Tory 1922 Committee's rules are changed to allow another confidence vote within 12 months.

Elections to the committee's ruling executive are expected next week, which could then lead to a decision on changing the rules.

West Dorset MP Chris Loder told BBC Radio 4's Today programme: "I think he does need to go.

"I think if he chooses not to, I think the 1922 Committee should act and I certainly would support that approach in the forthcoming 1922 elections."

Lee Anderson, one of the MPs elected in 2019 in Red Wall seats who largely owe their political careers to the Prime Minister, said he too had lost faith in the leader.

The Ashfield MP pointed to the row over Mr Pincher's appointment and said: "Integrity should always come first and sadly this has not been the case over the past few days."