Boris Johnson needs to “realise this is over” and quit as Prime Minister, the SNP’s leader at Westminster has said.

With the Conservative leader struggling to cling to power in Downing Street following a dramatic series of resignations, Ian 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.

However he appeared to rule out using any snap general election as a “de-facto referendum” on Scottish independence, saying this would go against the plan outlined by First Minister Nicola Sturgeon.

The “sequence of that” involves going to the Supreme Court first, Mr Blackford said, with judges having been asked to rule whether the Scottish Government can organise its own consultative ballot without UK consent.

If Supreme Court judges rule against the Scottish Government, Mr Blackford said “we will look at our tactics for an independence referendum using a general election”.

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.

In the wake of Mr Blackford’s comments, justice minister Victoria Atkins, Treasury minister John Glen and schools minister Robin Walker also resigned, while Tory MPs publicly revoked their support for the Prime Minister.

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.”

Meanwhile, Scotland’s Finance Secretary told the PA news agency that “while the UK Government is consumed with resignations and deceit”, the new Treasury team should focus on helping people struggling with the cost-of-living crisis.

When asked if a shift in Prime Minister could damage the ability of the UK to respond to the cost-of-living crisis, the Finance Secretary added: “At a time of crisis, the public needs to have confidence in their political leaders that they understand what truth is, they understand what integrity is and they understand that matters more than anything else at a time of crisis.

“Right now I would argue that the public, and I certainly, do not have confidence that at the very top of the UK Government there is any concept or any understanding of those vital concepts.”

The 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.”

Scottish Conservative leader Douglas Ross also made clear he has no confidence in Mr Johnson.

A party spokesman said: “Douglas made his position clear in the vote of no confidence against Boris Johnson. That remains his position.”

Lord Duncan said if the Tories can install a new leader in Number 10, the party could still do “good things”.

He said: “At the moment much of the concern is about an individual. If a new individual can come in, shake that up, with such a significant majority good things can still be done by this party in government.”