A former UK director of public prosecutions (DPP) has said Boris Johnsoncould face prison if he refuses to delay Brexit in the face of court action.

Ken MacDonald, who served as DPP from 2003 to 2008, said the prime minister could be found in contempt of court if he chooses to ignore a Bill which requires him to delay Britain leaving the European Union beyond October 31.

The legislation, The European Union (Withdrawal) (No.6) Bill, has been passed by the House of Lords and Queen Elizabeth is expected to sign it into law on Monday. It is designed to avoid a no-deal Brexit. 

“In conventional cases…individuals who are in contempt of court and fail to purge their contempt are liable to be committed to prison,” MacDonald, who now sits in the Lords, told Sky News.

David Lidington, who was deputy prime minister under Theresa May, said that obeying the rule of law was a fundamental principle of the ministerial code. “Defying any particular law sets a really, really dangerous precedent,” he told BBC radio.

Lidington resigned just before Johnson took office.

Dominic Grieve, a former attorney general and one of 21 Conservative lawmakers ousted from the party this week, said Johnson was unfit for office.

“This is ridiculous, it’s shaming, it’s like a four-year old having a tantrum,” he told Sky News.

Johnson has said the only solution to the Brexit deadlock is a new election, which he wants to take place on October 15 and which could give him a new mandate to quit the EU on schedule.

Two-thirds of parliament’s lawmakers need to back an early election, but opposition parties, including Labour, have said they would either vote against or abstain on this until the law to force Johnson to seek a Brexit delay is implemented.

Johnson failed to win enough support in a vote on Wednesday for an election. Another vote is scheduled for Monday. 

In the meantime, British lawmakers are preparing legal action in case Johnson tries to defy legislation compelling him to seek a further delay to Brexit, opposition Labour Party leader Jeremy Corbyn said on Saturday.

The BBC reported earlier that lawmakers, including moderate Conservatives expelled this week from their party for backing the bill, have lined up a legal team and are willing to go to court to enforce the legislation if necessary.

Corbyn said Labour was not as a party taking legal action but was aware of lawmakers’ manoeuvres on the matter.

The government had no immediate comment.

Johnson, a leader of the campaign to leave the EU during the 2016 Brexit referendum, took office in July after Conservative party predecessor Theresa May quit following three failed attempts to get a deal with Brussels through parliament.

Johnson has vowed to take Britain out of the EU on October 31 with or without a deal with the bloc.

He has said he has no intention of seeking an extension and would rather “die in a ditch” than delay Brexit.

Saturday’s Daily Telegraph newspaper reported that the prime minister is prepared to defy parliament’s instruction to request an extension to the Brexit process if he fails to agree a new deal.

The newspaper quoted Johnson as saying he was only bound “in theory” by the new legislation.

“We’re in quite extraordinary territory when the prime minister says he is above the law,” Corbyn told Sky News.