The Brexit Party has announced that it will not stand candidates in the 317 seats won by the Conservatives at the 2017 general election.
Party leader Nigel Farage said standing candidates across the country could increase the chances of another EU referendum taking place.
But he said the party would stand against all other parties – and focus on taking seats off Labour.
He had previously pledged to field more than 600 election candidates.
He said Boris Johnson’s commitment to strike a trade deal with the EU without “regulatory alignment” was a “huge change” in the Conservatives’ approach to Brexit.
The prime minister welcomed the move, calling it “a recognition that there’s only one way to get Brexit done, and that’s to vote for the Conservatives”.
But Tory chairman James Cleverly added there was still a “danger” the Brexit Party could split the vote in Tory target seats, leading to the election of MPs who could “frustrate the Brexit process”.
Labour Party chairman Ian Lavery said the move would create a “Nigel Farage and Boris Johnson alliance with Donald Trump to sell out our country”.
“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.”
Mr Farage had previously offered to not to stand candidates against the Tories in certain seats if the prime minister changed aspects of his Brexit deal.
But the proposal was rejected by Boris Johnson, who said deals with “any other party” would “risk putting Jeremy Corbyn into No 10”.
Will Brexit Party move help the Conservatives?
By BBC political analyst Peter Barnes
The Brexit Party’s decision to stand aside in constituencies the Conservatives won in 2017 should make it easier for them to hold on to those seats.
In marginals like St Ives and Mansfield, it ought to help them keep a bigger share of the pro-Leave vote.
However, to win a majority in the House of Commons the Conservatives need to win more seats – not just hold on to what they have.
And in those places the presence of a Brexit Party candidate may get in the Conservatives’ way.
The best estimates suggest that 29 of the Conservatives top 50 targets, and 59 of the top 100, are in Labour-held seats that voted Leave in the referendum.
If the strategy is to target Leave voters in those places, it would probably be better to have a clear run – without the Brexit Party.
Anti-Brexit parties Plaid Cymru, the Green Party and the Liberal Democrats have agreed not to stand against each other in 60 seats across England and Wales.
Their pact means that, in Wales, two of the parties will agree not to field a candidate, boosting the third candidate’s chances of picking up the Remain vote.
In England, it will simply be a two-way agreement between the Lib Dems and the Greens.
Scottish National Party leader Nicola Sturgeon said the Conservatives have “effectively become the Brexit Party”.
She added that defeating the Tories in Scotland “will help deprive Boris Johnson’s increasingly extreme and right-wing party of the majority they crave”.