Opposition parties have been urged to be ready to “think the unthinkable” and impeach Boris Johnson if he ignores laws designed to block a no-deal Brexit on October 31.

Plaid Cymru said Labour, the SNP, Liberal Democrats and other opposition MPs must consider previously “far fetched” ideas because the prime minister is “driving a bulldozer” through Britain’s constitution.

A majority of MPs would have to back impeachment for the process to begin, and it could end with Johnson being put on trial in parliament.

The House of Commons library describes impeachment as “obsolete” as it has been superseded by other forms of accountability and the rules “have not been adapted to modern standards of democracy or procedural fairness”.

But the PM has himself backed the idea in the past, in 2004 signing a motion proposed by Plaid leader Adam Price – then an MP – calling for the impeachment of Tony Blair over the Iraq war.

Johnson also wrote a Daily Telegraph column entitled: “Isn’t it time to impeach Blair over Iraq?”

Foreign Secretary Dominic Raab on Sunday suggested Johnson may try and circumvent the Benn bill, which compels him to seek a Brexit delay if he cannot win MPs’ approval for leaving with either a deal or no deal by October 19.

And Plaid’s Westminster leader Liz Saville Roberts said the opposition must be ready to “fight fire with water, outsmart the smartest, think the unthinkable”.

“Boris Johnson has already driven a bulldozer through the constitution, so no longer are ideas like impeachment far fetched,” she said.

“I will tell other opposition party leaders, we need to be ready to impeach Boris Johnson if he breaks the law.

“We cannot play the prime minister at his own cynical game. We need to be ready to fight fire with water, outsmart the smartest, think the unthinkable.

“Impeachment was a process backed by Boris Johnson not so long ago. A man sacked for serially lying backed the impeachment of Blair for the same reason – lying. If the prime minister becomes a law-breaker, we have an even stronger case for impeachment than the very cause he advocated back in 2004.

“No PM would want to play fast and loose with views he held on such matters in the past.

“The future of our communities are at stake and I will not let him get away with running them into the ground thanks to a plummy accent and a memorable haircut. 

“They have brought a campaigner to a constitutional fight, when they should have brought a lawyer.

“No one is above the law, Boris Johnson shouldn’t risk finding that out the hard way.”  

It only takes one MP to make the accusation of high crimes and misdemeanours against a public official for the impeachment process to begin, Plaid said.

Once the accuser has presented his or her proofs to the Commons and if MPs agree that there is a case to answer, a committee is established to draw up articles of impeachment. 

The House of Lords is notified and if they agree to the articles they then appoint prosecutors to try the case before peers, who are the judges. 

If there is a conviction the Commons decides the sentence.

Although a number of ministers up until the 19th century have been impeached, no prime minister has ever been successfully been impeached.