Nigel Farage has abandoned plans for the Brexit Party to contest more than 600 candidates in the General Election.
Speaking to supporters in Hartlepool he said the party would not stand in the 317 seats won by the Conservatives in the last election in 2017.
He said he had taken the decision because he feared that if they had run it would have led to a hung Parliament with significant gains for the Liberal Democrats.
The move is a significant boost for Boris Johnson amid warnings by Conservative ministers that they risked splitting the Brexit vote.
“We’ve decided ourselves that we absolutely have to put country before party and take the fight to Labour,” Mr Farage said.
The Brexit Party leaders had previously warned the Tories that it would stand in 600 constituencies unless Boris Johnson abandoned his Brexit deal with Brussels.
However, his offer of a “Leave alliance” electoral pact with the Conservatives was rejected by the Tories.
Mr Farage 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 by 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.
Mr Farage said: “Last night, I weighed up Boris’s promises, and is he going to stick to them against the threat particularly in the South and the South West that we let in a lot of Remainer Liberal Democrat MPs?
“I think our action, this announcement today, prevents a second referendum from happening.
“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.”
Mr Johnson, on the campaign trail in Wolverhampton, welcomed Mr Farage’s decision.
He told reporters that he had “absolutely not” called the Brexit Party leader 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 said.
Mr Farage had faced pressure from within the Brexit Party to back down amid fears that it could jeopardise the chances of leaving the EU by splitting the pro-Brexit vote.
His long-time ally – millionaire backer Arron Banks – had announced plans for a tactical voting app which would have urged voters in hundreds of constituencies to support the Tories.
On Monday, Mr Banks praised what he said had been a “brilliant decision” by his old friend.
“We will campaign hard to return Tory and Brexit party MPs! Nigel Farage putting country before party,” he tweeted.
Other opposition parties, however, were swift to condemn the move.
We urge voters to reject this Thatcherite 1980s tribute act, which would lead to more savage Tory attacks on working class communitiesIan Lavery, Labour
Labour Party chairman Ian Lavery said: “This is a Nigel Farage and Boris Johnson alliance with Donald Trump to sell out our country and send £500 million per week from our NHS to US drugs companies.
“We urge voters to reject this Thatcherite 1980s tribute act, which would lead to more savage Tory attacks on working class communities.”
Liberal Democrat leader Jo Swinson tweeted: “The Conservative Party are the Brexit Party now.”
Scottish First Minister Nicola Sturgeon said: “Any form of Brexit that is acceptable to Nigel Farage will be deeply damaging for Scotland.”