Brexit Party leader Nigel Farage says he will stand down more than 300 candidates in seats won by the Conservative Party in the previous UK general election.

Key points: The withdrawal of candidates marks a backdown for Mr Farage, who had threatened to field candidates in 600 seats

The withdrawal of candidates marks a backdown for Mr Farage, who had threatened to field candidates in 600 seats Mr Farage said the withdrawal of candidates was contingent on Boris Johnson not extending the Brexit transition period

Mr Farage said the withdrawal of candidates was contingent on Boris Johnson not extending the Brexit transition period Labour leader Jeremy Corbyn suggested Donald Trump had instigated the backflip

The move is a huge boost for Prime Minister Boris Johnson's chances at the December 12 poll and for his promise to deliver Brexit and take Britain out of the European Union.

Mr Farage said the decision was to put the country before his party and to ensure that Brexit would happen.

"The Brexit party will not contest the 317 seats the Conservatives won at the last election," Mr Farage said at a campaign rally in Hartlepool, which voted 70 per cent Leave in the 2016 referendum.

"We will concentrate our total effort into all the seats that are held by the Labour party, who have completely broken their manifesto pledge in 2017 to respect the result of the referendum, and we will also take on the rest of the remainer parties.

"We will stand up and fight them all."

Farage backtracks on candidate threat

The withdrawal of candidates marks a backdown for Mr Farage, who last week had threatened to field candidates in over 600 of the UK's 650 constituencies, potentially splitting the Leave vote and giving Remain-backing parties a leg-up.

Mr Farage said the plan was contingent on Mr Johnson keeping his promises, including not extending the Brexit transition period beyond the end of 2020 and trying to negotiate a trade deal with the EU that mimics Canada's current deal with no political alignment.

Mr Johnson welcomed the announcement, saying Mr Farage recognised a hung Parliament could prevent Brexit being accomplished and lead to another EU referendum and a referendum on Scottish independence.

"If we have another hung Parliament it would lead to two more chaotic referendums next year," Mr Johnson tweeted.

"The Conservatives only need nine more seats to win a majority and leave [the EU] by the end of January with a deal."

"We can then finally move on as a country, and focus on the priorities that matter to you and your family."

A 'Trump alliance'

Jeremy Corbyn criticised the move, calling it a "Trump alliance" that "must be stopped". ( AP: Kirsty Wigglesworth )

Opposition parties were quick to criticise the move, with many saying the Brexit Party was now an ally of the Conservatives.

Labour leader Jeremy Corbyn called the move a "Trump alliance" that "must be stopped".

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"One week ago Donald Trump told Nigel Farage to make a pact with Boris Johnson," he tweeted.

"Today, Trump got his wish.

"This Trump alliance is Thatcherism on steroids and could send GBP 500 million a week from our NHS to big drugs companies. It must be stopped."

Liberal Democrat leader Jo Swinson, whose party was tipped to benefit the most from Brexit Party candidates taking on Conservatives, tweeted "The Conservative Party are the Brexit Party now".