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UPDATE 4-British lawmakers prepare court action to enforce Brexit delay

* New law compels PM to seek further Brexit postponement

* Lawmakers preparing possible legal action against Johnson

* Johnson says only bound by new law "in theory" -report

* Labour leader Corbyn says Johnson must abide by law

* Former prosecutor says Johnson could face jail -Sky News (Adds detail, comments from lawmaker Dominic Grieve, former public prosecutor)

LONDON, Sept 7 (Reuters) - British lawmakers are preparing legal action in case Prime Minister Boris Johnson tries to defy legislation compelling him to seek a further delay to Brexit, opposition Labour Party leader Jeremy Corbyn said on Saturday.

An opposition bill which would force Johnson to ask the European Union for an extension to Britain's departure to avoid an Oct. 31 exit without a transition deal was approved by parliament's appointed upper chamber, the House of Lords, on Friday.

Queen Elizabeth is expected to sign it into law on Monday.

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 Oct. 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.


'HAVING A TANTRUM'

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.

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

"In conventional cases...individuals who are in contempt of court and fail to purge their contempt are liable to be committed to prison," Ken MacDonald, who served as DPP from 2003 to 2008 and now sits in the Lords, told Sky News.

David Lidington, who was deputy prime minister under 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.

Johnson has said the only solution to the Brexit deadlock is a new election, which he wants to take place on Oct. 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.

An opinion poll on election voting intentions, carried out by Survation for the Daily Mail newspaper, put the Conservatives on 29%, down 2 percentage points from the previous poll, with Labour unchanged on 24%. The pro-EU Liberal Democrats were on 18% and the Brexit Party on 17%.

Separately on Saturday, the British Chambers of Commerce said a "concerningly high number" of firms were not ready for a no-deal Brexit.

Its survey of 1,500 firms found 41% had not even done a Brexit risk assessment. “Our evidence yet again reinforces the importance of averting a chaotic exit on Oct. 31," Director General Adam Marshall said. (Reporting by James Davey; editing by Mark Heinrich and Jason Neely)