Nigel Farage has abandoned plans for the Brexit Party to contest more than 600 candidates in the forthcoming 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.
Mr Farage said: "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.
"We've decided ourselves that we absolutely have to put country before party and take the fight to Labour."
Mr Farage said Mr Johnson's move towards a free trade deal with the European Union that did not include regulatory alignment was a "significant change" to the approach on Brexit.
"He said we would negotiate a super Canada-plus trade deal with no political alignment," said the Brexit Party leader.
"That is a huge change. Ever since Mrs May's abject speech in Florence, we have been aiming at a close and special partnership with the European Union.
"We had been aiming to stay part of many of its agencies.
"Boris last night signalled a very clear change in direction. I thought to myself overnight, 'That sounds a bit more like the Brexit we voted for'."
Following today's announcement, Scottish First Minister Nicola Sturgeon tweeted: "Any form of Brexit that is acceptable to Nigel Farage will be deeply damaging for Scotland.
"Makes it all the more important to get rid of Boris Johnson's Tories, escape Brexit and put Scotland's future into Scotland's hands."
Liberal Democrat leader Jo Swinson tweeted: "The Conservative Party are the Brexit Party now."
Supporters in the room backed Nigel Farage's announcement, calling him a "good strategist".
Brexit Party supporter 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."
His son Christopher (39) added: "I'm pretty pleased he has done it.
"We want out and that seems to be the best way of going about it, as long as Boris can keep his word."
