PM Boris Johnson will call an election for October 14 if MPs vote against his Brexit strategy. (File)

British Prime Minister Boris Johnson will call an election for October 14 if MPs vote against his Brexit strategy, a top official said Monday on the eve of a parliamentary showdown.

MPs from Johnson's own Conservative Party are preparing to join opposition lawmakers in a vote on Tuesday to try to force him to delay Brexit if he cannot strike an agreement with Brussels in the next few weeks.

A senior official said that if the government loses the vote, it would table a parliamentary vote for Wednesday on holding an early election.

Under British law, a two-thirds majority is required to hold early elections but the main opposition Labour Party has already said it would be in favour.

The poll would take place on October 14, to allow a new government to be in place before Britain is scheduled to leave the European Union on October 31.

"The prime minister doesn't want an election but that will be up to MPs when they choose to vote tomorrow," the official said.

Johnson earlier expressed hope he could get a deal with Brussels on ending Britain's 46-year-old membership of the EU, but added that he would stick to the Brexit deadline.

"We are leaving on October 31, no ifs or buts," he said in a defiant statement outside his Downing Street office.

A cross-party group of MPs is seeking to change the law to potentially delay Brexit until January 31, 2020.

They fear that leaving the EU with no deal could cause huge economic disruption.

But Johnson took office in July promising to deliver the 2016 referendum vote for Brexit whatever the circumstances, and after talks with his top ministers on Monday evening, made clear he would not move.

He said the chances of striking a divorce deal with Brussels "have been rising" ahead of a summit of EU leaders on October 17 and 18, adding this was partly because the bloc understood he would walk away.

"I want everybody to know -- there are no circumstances in which I will ask Brussels to delay," Johnson said.

"Armed and fortified with that conviction I believe we will get a deal at that crucial summit in October."

Last chance

More than three years have passed since Britons voted by 52 percent to leave the European Union, a period marked by huge political upheaval, economic uncertainty and deep divisions both in parliament and across the country.

Former prime minister Theresa May agreed exit terms with Brussels last year but the deal, which covered Britain's financial contributions, the rights of EU expatriates and the Irish border, was rejected by parliament thrice.

Johnson has repeatedly called on the EU to renegotiate but it has so far refused, prompting both sides to ramp up preparations for a disorderly divorce.

Many MPs are opposed to "no deal" but they have only a few days to act.

They return from their summer holiday on Tuesday, and Johnson has controversially decided to suspend parliament next week for more than a month.

"We must come together to stop no-deal," Labour leader Jeremy Corbyn said in a speech on Monday, warning: "This week could be our last chance."

On Tuesday afternoon, a cross-party group of MPs will seek to take control of the parliamentary timetable in order to allocate time on Wednesday to debate their draft law blocking "no deal".

Their bill says that if MPs have not approved an EU deal or endorsed a "no deal" scenario by October 19 -- the day after the EU summit -- then Johnson must seek a delay to Brexit.

However, the coalition behind the plan is divided.

Corbyn said that if the legislative efforts failed, Labour could back a vote of no-confidence in Johnson's government, which could trigger a general election.

Opinion polls suggest Johnson's decisive action on Brexit is popular with voters, but an election -- particularly before Brexit is delivered -- could be a huge risk.

The 2016 referendum vote shook up the political landscape, with the governing Conservatives challenged not just by Labour, but also the eurosceptic UKIP and the pro-European Liberal Democrats.