Nigel Farage said his Brexit Party will not contest the 317 seats won by the ruling Conservatives in 2017.
The move, Mr Farage said, will avoid damaging Prime Minister Boris Johnson's efforts to secure a parliamentary majority to end the impasse over leaving the European Union.
Mr Farage says he wants to block the Liberal Democrat “fanatics” and prevent a second referendum on Brexit.
"The Brexit Party will not contest the 317 seats the Conservatives won at the last election," he said on the campaign trail in Hartlepool, north-east England.
“But what we will do is concentrate our total effort into all of the seats that are held by the Labour Party, who have completely broken their manifesto in 2017."
"Our action prevents a second referendum from happening," Mr Farage said. "We will also take on the rest of the remainer parties."
Mr Johnson welcomed the move, calling it "recognition that there's only one way to get Brexit done, and that's to vote for the Conservatives".
Leader of the opposition, Jeremy Corbyn, was less enamoured with the plan, saying it was influenced by US President Donald Trump.
The Labour leader tweeted “one week ago Donald Trump told Nigel Farage to make a pact with Boris Johnson … today, Trump got his wish.”
He called the move a “Trump alliance” that would allow US drug companies to get a foothold in the UK's National Health Service.
Others have expressed doubts over the effectiveness of Mr Farage's pledge.
John Curtice, professor of politics at Strathclyde University, said the move is “not offering the Conservatives much help” except in seats targeted by the Liberal Democrats. The gains made by the pro-EU party via its pro-Remain electoral pact with the Greens and Plaid Cymru — covering about 60 seats — have been “reversed” by the Brexit Party move, he said.
The announcement will be a significant boost for Mr Johnson before a December 12 election.
There are a total of 650 seats up for grabs.
