Brexit Party leader Nigel Farage has reneged on his promise to contest 600 seats at the next general election.
Mr Farage said his party will not contest the 317 seats won by the Conservatives at the 2017 general election.
Instead, they will put their "total effort" into contesting seats held by the Labour Party and "Remainer parties".
Mr Farage said he had taken the decision because he feared that if they had run against the Conservatives it would have led to a hung Parliament with significant gains for the Liberal Democrats.
And he said a pledge from Boris Johnson to rule out extending the transition period beyond 2020 and the promise of a Canada-plus-style free trade agreement was "optimistic".
The move is a significant boost for Boris Johnson amid warnings by Conservative ministers that Mr Farage's party risked splitting the pro-Brexit vote.
"We've decided ourselves that we absolutely have to put country before party and take the fight to Labour," Mr Farage added.
It comes just two weeks after Mr Farage said his party would "contest every seat" unless the Prime Minister agreed to form an electoral pact with them.
However, the Conservatives repeatedly rejected offers to form a Leave alliance.
The Prime Minister insisted the “only way” to deliver Brexit was to vote Conservative.
Meanwhile, the Liberal Democrats have teamed up with the Green Party and Plaid Cymru in a bid to keep Britain in the EU.