MPs should be ready to impeach Boris Johnson if he tries to break the law by pushing through a no-deal Brexit, an opposition party leader has said.

Liz Saville Roberts, the Westminster leader of Plaid Cymru, said the arcane device could be used to remove the prime minister from office.

It comes amid reports that Mr Johnson will force a legal battle rather than ask the EU to delay Brexit, as parliament has ordered him to do if no deal in place by 19 October. No10 sources have suggested the prime minister could try to “sabotage” the law.

According to the House of Commons library, the last MP to face impeachment was Viscount Melville in 1806, although the attempt to impeach him for misappropriating public money was unsuccessful.

The device has since been replaced by other methods of holding MPs for account.

But Plaid Cymru said it remained legally valid to impeach MPs “for high-treason and other crimes and misdemeanours” and could be used against Mr Johnson.

The current prime minister previously called for the same technique to be used against Tony Blair.

Writing in The Telegraph in 2004, the then Conservative backbencher said Mr Blair had ”treated parliament and the public with contempt” and therefore deserved “to be impeached”.

He also signed a parliamentary motion demanding that a committee of MPs be set up to consider “articles of impeachment” against Mr Blair.

According to parliamentary conventions, it takes only one MP to force a vote on impeaching another MP.

The whole House of Commons would then vote on whether a trial, which would be conducted jointly by the Commons and the Lords, should take place. The final decision on whether the MP was guilty would be taken by the Lords.

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Ms Saville Roberts said she would urge other party leaders to be ready to impeach Mr Johnson, saying the prime minister had “already driven an bulldozer through the constitution”.

She said" “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.”