HARTLEPOOL, England (Reuters) - Prime Minister Boris Johnson won the most significant boost of the British election campaign to date when Nigel Farage’s Brexit Party pulled out of 317 Conservative-held seats in a bid to prevent opponents of Brexit controlling the next parliament.

The move dramatically increases the chances that Johnson will stay prime minister, and then finally deliver on the 2016 referendum result to take Britain out of the European Union.

Farage said Brexit was in peril, so he would focus his party’s efforts at unseating opponents of leaving the EU - primarily in seats held by the Labour Party and the Liberal Democrats.

The United Kingdom voted by 52%-48% in 2016 to quit the EU, albeit without deciding how it would be done. But parliament has been deadlocked since a 2017 snap election over how, when and even whether to leave.

A week ago, Farage had promised to field 600 candidates unless Johnson went for a “no-deal” Brexit.

But on Monday, he said his party will now “concentrate our total effort into all of the seats that are held by the Labour Party, who have completely broken their manifesto in 2017,” he said. “We will also take on the rest of the ‘remainer’ parties.”

Sterling leapt as much as 1% against the U.S. dollar GBP=D3 after Farage's announcement, and hit a six-month high against the euro GBPEUR=. [GBP/]

The Brexit Party’s threat to Johnson was one of the most unpredictable elements of the election: Farage has transformed British politics over a decade by poaching Conservative voters to force a succession of prime ministers towards ever tougher positions on Europe.

But in what amounts to a non-aggression pledge and an endorsement of sorts from the man cast by supporters as the godfather of Brexit, Farage spared Johnson the prospect of a right-wing challenge in almost half Britain’s 650 constituencies.

His Brexit Party will still run in seats held by the opposition Labour Party, opening up the possibility that he could yet be a kingmaker with just a few seats in a hung parliament.

Although the Conservatives are hoping to seize traditional Labour seats in northern England that are heavily pro-Brexit, analysts had said Farage posed a bigger threat to the Conservatives than to Labour.

BACKING FOR JOHNSON’S APPROACH

Johnson welcomed Farage’s move, which rivals said made the Conservatives in effect an ally of Farage’s hard-Brexit party.

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Opposition Labour Party leader Jeremy Corbyn said the move meant Farage’s friend U.S. President Donald Trump had got his wish - shared on a live radio interview with Farage - for an alliance between Farage and Johnson.

Farage said Trump would be “very, very pleased” to hear of the move, stemming from Johnson’s promise to go for a Canada-style trade deal with the EU.

Sara Hobolt, a professor who specialises in polling at the London School of Economics, said: “If I was Boris Johnson, I would be pretty pleased by this.”

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Farage began the campaign last week by warning Johnson that he could snatch away a Conservative victory. But he came under intense pressure from financial backers who feared that Farage might scupper Brexit altogether.

After weeks of complaining about Johnson’s Brexit deal, he said a video showing Johnson pledging not to extend the post-Brexit transition period beyond the end of 2020 had changed his mind.

“I thought to myself overnight, that actually sounds a bit more like the Brexit that we voted for,” said Farage, 55.

It was a rare compromise for a man who has made a career out of forcing the hand of Conservative prime ministers.

The perceived electoral threat to the Conservatives from the United Kingdom Independence Party that Farage used to lead was one of the main reasons why then-prime minister David Cameron decided in 2013 to promise the Brexit referendum.

Shunned by the political establishment, Farage, backed by eurosceptic financiers, helped sell Brexit to voters in England and Wales who felt ignored by Conservative and Labour alike.