Boris Johnson could soon be removed as prime minister under a plan being pushed by opposition parties.

Johnson's opponents fear that he will find a way to force Britain out of the European Union without a deal next month.

The Scottish National Party now believes the only way to prevent this is to make the Labour Party leader, Jeremy Corbyn, a caretaker prime minister.

Under the plan, Corbyn would become prime minister for a few weeks to delay Brexit and call a general election.

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Boris Johnson could soon be forced out as prime minister to make way for the Labour Party leader, Jeremy Corbyn. Here's why.

Johnson is the leader of a minority government, following the defection of one former Conservative MP to the Liberal Democrats and Johnson's decision to expel 21 members of his own party.

Johnson's advisers originally believed this would only be temporary and that opposition parties would swiftly vote for a general election, which some polls have suggested he would win.

However, the opposition has other ideas, blocking a general election until Brexit has been delayed beyond its October 31 deadline.

Johnson is running out of road

Jeremy Corbyn and Johnson. Getty

This presents a problem for the prime minister, who insists he will not delay Brexit, despite members of Parliament passing a law instructing him to do so.

This means Johnson may ultimately have little choice but to resign. However, Johnson's allies are saying he would take a different course and find some way to circumvent the Brexit-delay law.

John Major, a former Conservative prime minister, warned on Thursday that this could be exactly what Johnson is planning.

As a result, the UK's opposition parties are growing nervous and believe they may have to act first to prevent Johnson from finding some method of forcing Britain out of the European Union next month.

The caretaker prime minister 'has to be Corbyn'

Corbyn and Nicola Sturgeon. Getty

The Scottish National Party, the UK's third-largest party, now believes that the only surefire way to prevent Johnson from forcing through a no-deal Brexit is to remove him from office and replace him with Corbyn as a caretaker prime minister.

Under the plan, Corbyn, the leader of the second-largest party in Parliament, would briefly enter Downing Street with the sole purpose of delaying Brexit, before triggering a general election.

Responding to a tweet on Friday suggesting that opposition parties should temporarily install Corbyn as prime minister, Scottish First Minister Nicola Sturgeon replied, "Agree with this."

Sturgeon added that "nothing is risk free but leaving Johnson in post to force through no deal - or even a bad deal - seems like a terrible idea to me."

One senior SNP source told ITV's Robert Peston that Corbyn would be the only realistic choice for the role.

"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 source said.

"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."

Johnson's opponents see Corbyn as a lesser risk

Corbyn. Getty

Winning a vote in the UK Parliament to make Corbyn prime minister, however, looks tricky.

Even with the SNP's support, Corbyn would still need the votes of all other opposition parties as well as a significant number of former Labour and Conservative MPs to become prime minister.

Given that some of those former Labour MPs left the party specifically because of their opposition to Corbyn, this looks like a very difficult task. The UK's fourth-largest party, the Liberal Democrats, is also reluctant to install Corbyn even for a short period, with its leader, Jo Swinson, describing him as "not fit" to be the prime minister.

However, time is running out to prevent a chaotic exit from the European Union.

And if push comes to shove, Corbyn's critics may ultimately decide that a few weeks of the Labour leader in charge is less risky than allowing Johnson to remain there any longer.