Video report by ITV News Correspondent Emma Murphy
Nigel Farage has handed Boris Johnson a General Election boost after he dropped plans to stand Brexit Party candidates in more than 300 of the 600 constituencies he had targeted.
The Brexit Party leader said he had taken the "difficult decision" not to contest the 317 seats held by the Tories at the last election amid fears it could lead to a hung parliament and a second referendum.
The announcement was welcomed by the Prime Minister who said it was recognition that only the Conservatives could "get Brexit done".
However, opposition parties said it showed the Tories and the Brexit Party were now "one and the same".
Mr Farage had previously threatened to stand candidates in some 600 seats unless Mr Johnson abandoned his withdrawal deal with the EU.
But after the Conservatives rejected his offer of a "Leave alliance" he came under intense pressure from within his own party not to risk splitting the pro-Brexit vote.
In a tweet Mr Farage defended his move, sharing a letter which he said shows he is "putting country before party" in backing Boris Johnson.
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.
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Addressing a Brexit Party rally in Hartlepool, Mr Farage said he had decided to pull back amid concerns his party could have let in significant numbers of Liberal Democrats - opening up the prospect of a second referendum.
"I think this announcement today prevents a second referendum from happening," he said.
"And that to me, I think 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."
Speaking to ITV News later on Monday, Mr Farage explained his decision: "Simply, we've got to put country before party, it was a difficult decision, it was a very difficult decision but when the Prime Minister signalled a chance in direction, it became easier."
At the rally the Brexit Party leader said he still believed the Withdrawal Agreement negotiated with Brussels would not deliver "the Brexit we voted for" in the 2016 referendum.
However, he said he had been encouraged by statements from Mr Johnson at the weekend saying he would not extend the planned transition period beyond the end of 2020 and that he would seek a "super-Canada-plus" style free trade agreement with the EU.
ITV News Correspondent Emma Murphy was watching the speech in Hartlepool and said most of those in the room supported Mr Farage's decision.
But many believe it could still split the vote in Tory target seats, as the Brexit Party takes the fight to Labour.
Polling experts said the Brexit Party move would help the Tories in seats where they were under threat - particularly from the Lib Dems.
However, they will still be contesting Labour-held seats which the Tories need to win if they are to gain an overall majority in the poll on December 12.
Chris Curtis, head of political research at pollsters YouGov, said: "It is still the case that most marginal seats are Labour-Conservative battles and this is the most important dynamic in deciding who will be celebrating Christmas in 10 Downing Street.
"Given this, Farage's decision to stand aside in current Conservative-held seats and not in Labour-held seats that the Tories will be looking to gain will likely make very little difference."
Nevertheless, the move was still welcomed by Mr Johnson, who flatly denied that he had called Mr Farage to agree a deal.
"I'm glad that there's a recognition that there's only one way to get Brexit done and that's to vote for the Conservatives," he told reporters on the campaign trail in Wolverhampton.
Labour leader Jeremy Corbyn said Mr Farage's move could pave the way to a US trade deal, giving American pharmaceutical companies access to the NHS.
"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," he said.
Fact checking charity Full Fact said Mr Corbyn's £500 million claim is "fairly extreme", but added that "drug prices could increase following a trade deal".
Lib Dem deputy leader Sir Ed Davey told ITV News: "Voters now know if they vote Conservative they are voting for Nigel Farage and the Brexit Party.
"There will be many Conservative voters worried to back such an extreme position.
"Now Boris Johnson has the endorsements of Donald Trump and Nigel Farage - that will make many people think twice about voting for such a hard-right Tory Brexit party."
He went on to say Mr Farage's announcement could indeed help his party to secure more seats, pointing to what he believes is a Conservative voter reluctance to support the Brexit Party leader.
Scottish First Minister Nicola Sturgeon added: "Any form of Brexit that is acceptable to Nigel Farage will be deeply damaging for Scotland."
Elsewhere, a Green Party candidate has tactically withdrawn with the aim of helping Labour to unseat Iain Duncan Smith in Chingford & Woodford Green.
John Tyne, the Green Party candidate for Chingford & Woodford Green, said: "The actions of IDS as the architect of the failed rollout of Universal Credit mean that we believe he is unfit for office.
"The party believes that its withdrawal from this election offers the best chance for voters to register a concerted vote against him and his failed policy."
Brexit Party supporters shared mixed feelings over the announcement.
Rachel Swann, 48, from Redcar, said: "It would be great to see him contesting every seat but this is logical, a lot of people are concerned we are splitting the Leave vote.
"He is a good strategist, it's one I can live with."
Robert Kenny, 64, from Hartlepool, watched the speech and said afterwards: "I think he got a bit of a surprise, he might have thought the reaction in the room was going to be against."