BORIS Johnson has been handed an unexpected General Election boost after Nigel Farage announced the Brexit Party would not contest 317 Tory-held seats but concentrate its fire on Labour.

Mr Farage said he had given the Prime Minister an opportunity to win a Commons majority, saying: “I’ve given him a chance to defend the ones he’s got already; he doesn’t need many more to win a majority; everyone knows that.”

While the move does not guarantee the Tory leader will win the election outright, it helps provide the basis for the Conservatives to concentrate their campaign more on winning Labour-held seats in the Midlands and northern England and so boosts Mr Johnson’s chances of getting back into Downing St on December 13.

Last night, senior Conservatives were calling on the Brexit Party leader to go further and pull out more candidates from Tory-Labour marginals.

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One UK Cabinet minister said: “Farage accepts that a vote for the Brexit Party is a vote for Corbyn and a second referendum. But he hasn’t pulled out of the seats we need to win to defeat Corbyn and deliver Brexit.”

Mr Farage explained he had performed the U-turn - only last week he insisted the Brexit Party would field 600 candidates - after the PM made clear at the weekend there would be no extension to the post-Brexit transition period beyond December 2020 and that he wanted a "super-Canada-plus" style free trade agreement with the EU.

The anti-EU MEP told supporters that it was time to “put country before party and take the fight to Labour”.

He said: “The Brexit Party will not contest the 317 seats the Conservative Party won at the last election but what we will do is concentrate our total effort into all of 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 can also take on the rest of the Remainer parties.”

The Prime Minister, on the campaign trail in the Midlands, declared: "I'm glad there's a recognition that there's only one way to get Brexit done. And that's to vote for the Conservatives." He flatly denied he had called Mr Farage to agree a campaign deal.

However, Nicola Sturgeon warned Mr Farage’s decision showed Mr Johnson was “having his strings pulled” by the Brexit Party leader and that Scotland’s future was now at risk of being decided by the anti-EU MEP.

“The Brexit Party's announcement really does underline the fact Boris Johnson and Nigel Farage are joined at the hip and, frankly, any form of Brexit that is acceptable to Nigel Farage is going to be deeply damaging for Scotland,” declared the First Minister.

"I suspect there are many traditional Tory Party voters here in Scotland and elsewhere across the UK who will be appalled to learn their party has effectively become the Brexit Party.

"If you vote Tory, you get the Brexit Party and its view of the world. I don't think that's what the vast majority of the people in Scotland want,” she added.

Sir Ed Davey for the Liberal Democrats said: "Nigel Farage standing down shows the Conservatives and the Brexit Party are now one and the same."

Naomi Smith, Chief Executive of the pro-EU campaign group Best for Britain, declared: "Farage has bottled it and hung most of his own candidates out to dry."

She added: "It's now more important than ever Remainers use their votes wisely. Our best chance of stopping a nightmarish government delivering a hard and damaging Brexit is voting tactically."

Jeremy Corbyn, who will make his first campaign visit to Scotland in the next 24 hours, tweeted his response: "One week ago Donald Trump told Nigel Farage to make a pact with Boris Johnson. Today, Trump got his wish.

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

Today, the Tories will keep up their attack on Mr Corbyn’s “reckless” spending plans after claiming on Sunday that they would amount to £1.2 trillion over five years; a charge Labour denounced as “ludicrous nonsense”.

Sajid Javid will reveal more Conservative research claiming Labour’s plans could hand an “additional £2,400 bill to every taxpayer every year” if Mr Corbyn became PM.

“This figure - the ‘cost of Corbyn,’” declared the Chancellor, “was equivalent to an entire month’s pay for the average earner.”

But Jonathan Reynolds for Labour dismissed the Tory figures as “more fake news” and stressed his party would set out its fully-costed plans when it published its manifesto, expected next week.

"The Conservatives should spend more time looking at their own policies as they failed to produce costings at the last election and, as Kwasi Kwarteng[the Business minister] demonstrated, have no idea about the cost of their own policies," added the Shadow Treasury Minister.

Yesterday during a rally in Leave-voting Hartlepool, Mr Farage took the election campaign by surprise and appeared to bow to pressure from within his own party not to split the pro-Brexit vote after the Conservatives rejected out of hand his offer of a "Leave Alliance".

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His long-time ally - millionaire backer Arron Banks - announced plans for a tactical voting app, which would have urged voters in hundreds of constituencies to support the Tories.

Mr Farage told supporters he had decided to pull back amid concerns his party could have let in significant numbers of Liberal Democrats, who have formed a Remain Alliance with the Greens and Plaid Cymru, opening up the prospect of a second referendum.

“This announcement today prevents a second referendum from happening," declared the party leader. And that to me, right now, is the single most important thing in our country.

"So in a sense we now have a Leave alliance, it's just that we've done it unilaterally. We've decided ourselves that we absolutely have to put country before party and take the fight to Labour," he added.

However, not all the Brexit Party candidates were happy with the decision.

Neil Greaves, from the Harlow constituency in Essex, has decided to defy his party leader and stand as an independent Brexiteer. He said Mr Farage had been "totally outmanoeuvred" and encouraged his fellow candidates to follow his example.

"Nigel has let Brexiteers down,” declared the 52-year-old. “He should be standing up for the 17.4 million people who voted for Brexit. I don't regard Boris Johnson's deal as Brexit. It's not even close.”

In Scotland, five Brexit Party candidates were down to stand in Tory-held seats. One, Les Durance, who was contesting Moray, said: "I am disappointed with the announcement. I put a lot of work in over in Moray, which voted by nearly 50 per cent to Leave the European Union in 2016.

"I do understand Nigel's rationale though; he has put country before party. In Moray I suppose the prognosis was we would have split the vote and possibly allowed the SNP to take the seat. It is an enormous concession and I just hope that Boris honours his commitments."

Mr Durance said he would now switch to contest the SNP-held seat of Inverness, Nairn, Badenoch and Strathspey.