The Scottish National Party has come round to the idea that Jeremy Corbyn may shortly have to become temporary caretaker prime minister, in order to prevent a no-deal Brexit on 31 October and immediately afterwards hold a general election.

A source close to the SNP leadership tells me that Ian Blackford, leader of the SNP in Westminster, and Nicola Sturgeon, Scotland's First Minister, are deeply concerned that it may now be impossible to prevent a no deal Brexit unless Boris Johnson is removed from office.

One said: "It is increasingly clear that we will have to install a new prime minister via a vote of no confidence, so that we can request a delay to Brexit and hold an election. The convention is absolutely clear that it is the leader of the opposition - in this case Jeremy Corbyn - who should become prime minister in those circumstances.

"Trying to find a compromise candidate, a national unity candidate, is too complicated, especially in the time we have. Whether people like it or not, the temporary prime minister has to be Corbyn."

Former prime minister John Major. Credit: PA

The source added that the other opposition parties, and the 34 independent MPs, would have to come round to backing Corbyn, if they want to be certain of avoiding a no deal Brexit.

For the Lib Dems, led by Jo Swinson, seeing Corbyn as PM even for a short period may be too much to swallow. The 21 former Tory MPs expelled by Johnson for backing the Brexit-delaying Benn Act, and the former minister Amber Rudd who resigned from the Cabinet in solidarity with them, are also deeply hostile to the idea of Corbyn becoming PM, even for a few weeks.

But according to a senior SNP member, the threats from Johnson and his senior aide Dominic Cummings that the Benn Act will prove ineffectual in blocking a no deal Brexit have to be taken "extremely seriously".

Dominic Cummings. Credit: PA

One senior SNP MP told me that the party agrees with Sir John Major, the former Tory minister, who warned tonight that Boris Johnson may use an order of council to delay the implementation of the Benn Act, the legislation forced on the PM by MPs whose purpose is to compel Johnson to request a Brexit postponement if he has not secured a new Brexit deal by 19 October.

In a speech at residence of the Spanish ambassador, Major said:

"It is important to note that an Order of Council can be passed by Privy Councillors – that is Government Ministers – without involving HM The Queen. I should warn the Prime Minister that – if this route is taken – it will be in flagrant defiance of Parliament and utterly disrespectful to the Supreme Court. It would be a piece of political chicanery that no-one should ever forgive or forget."

The combined forces of Labour and the SNP alone are not sufficient to bring down Johnson. And the motives of the SNP in swinging towards support for a temporary Corbyn premiership will be mistrusted by other opposition MPs, because opinion polls show that the SNP would win by a landslide in Scotland in any imminent general election.

But opposition MPs are increasingly united in their unease at the way that Johnson is using emotional and provocative language - such as describing the Benn Act as the "Surrender" Act - to generate hostility against parliament among those who backed Brexit in the referendum.

Every opposition leader has described Johnson as "unfit to govern" since the Supreme Court ruled on Tuesday that his five-week prorogation or suspension of parliament was unlawful.