Nigel Farage has said the Brexit Party will not contest Conservative seats in the upcoming general election.
The Brexit Party leader said it would not put up candidates in the 317 seats secured by the Tories in 2017.
The move marks a sharp U-turn after Mr Farage previously said his party would stand candidates in more than 600 seats, and is a huge boost for Boris Johnson.
The prime minister said he had "absolutely not" called Mr Farage to agree a deal.
He said: "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."
Mr Farage said: "We've decided ourselves that we absolutely have to put country before party and take the fight to Labour.”
His decision comes after polls showed support for the Brexit Party have plummeted since the election was called.
"I will tell you now exactly what we are going to do," he told supporters in Hartlepool. "The Brexit Party will not contest the 317 seats the Conservatives won at the last election.
"But what we will do is concentrate our total effort into all 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 will also take on the rest of the Remainer parties. We will stand up and we will fight them all."
Mr Farage said the prime minister'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'."
Mr Farage said he still wants to see Brexit Party MPs elected to hold Mr Johnson to account if he wins a majority.
"It's not easy, but how do we hold Boris to his promises?,” Mr Farage said. “That's the key to this, isn't it? That's the key to whether this strategy actually works."
He added: "The way we keep Boris Johnson to his promises is we have to start to win some of these seats. That's what we need to do in this election.
"Because when we do that and we have a Brexit Party voice in Parliament, we're going to keep saying, 'Remember, you told us we were leaving by the end of 2020. Remember, you told us we're not going to have political alignment.’
"And actually he'll know, just as Mrs May's vote disappeared in the European elections of this year, the same will happen again if a British prime minister breaks firm commitments."
Mr Farage added: "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."
Mr Farage continued: "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.”
What is the reaction to Nigel Farage’s decision?
Boris Johnson said the Conservative Party welcomed Nigel Farage's "recognition that another gridlocked hung Parliament is the greatest threat to getting Brexit done".
He wrote: "If we have another hung Parliament it would lead to two more chaotic referendums next year.
“The Conservatives only need nine more seats to win a majority and leave by the end of January with a deal.
Only a Conservative majority can get Brexit done by the end of January with a deal that's agreed and ready to go. pic.twitter.com/u5BUvfG3Wk— Boris Johnson (@BorisJohnson) November 11, 2019
Labour leader Jeremy Corbyn tweeted: "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."
"We can then finally move on as a country, and focus on the priorities that matter to you and your family.”
One week ago Donald Trump told Nigel Farage to make a pact with Boris Johnson.— Jeremy Corbyn (@jeremycorbyn) November 11, 2019
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.
Responding to Mr Farage's decision, Liberal Democrat leader Jo Swinson tweeted: "The Conservative Party are the Brexit Party now."
Labour election candidate and former minister David Lammy tweeted: "Nigel Farage bottling it by standing down in Tory seats shows how vital it is for Remainers to cooperate.
"We cannot allow this hard right alliance between Boris Johnson and Nigel Farage – dancing to the tune of Donald Trump – to permanently wreck our country."
The Conservative Party are the Brexit Party now. https://t.co/ijpWUUMDvh— Jo Swinson (@joswinson) November 11, 2019
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."
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. #GE209 #VoteSNP https://t.co/3M5C4TUnDJ— Nicola Sturgeon (@NicolaSturgeon) November 11, 2019
Labour Party chair Ian Lavery said the Brexit Party’s decision to stand down candidates in Tory-held seats was part of a bid to satisfy Donald Trump and privatise parts of the NHS.
"This is a Nigel Farage and Boris Johnson alliance with Donald Trump to sell out our country and send £500m per week from our NHS to US drugs companies," he said.
"We urge voters to reject this Thatcherite 1980s tribute act, which would lead to more savage Tory attacks on working class communities. Our NHS is not for sale."
Green Party election candidate and former leader Caroline Lucas tweeted: "Nigel Farage doesn't need to stand against Conservatives because under Boris Johnson, the Conservatives have morphed into the same narrow sect as Brexit Party.
"This is the alliance Trump wanted. We must resist delivering Trump the result he wants too."