The Liberal Democrats have confirmed they will form a pact with the Remain-backing Green Party and Plaid Cymru in the December general election.
Jo Swinson, the party’s leader, billed the agreement as a “significant moment” for voters across the country who want Brexit to be stopped.
Speculation over a so-called Remain alliance between had been growing over recent weeks.
Confirming that it had been formed, Lib Dem leader Ms Swinson said: "We are delighted that an agreement has been reached. This is a significant moment for all people who want to support remain candidates across the country."
Between 60 and 70 constituencies are expected to be covered by the deal.
The arrangement was brokered by the Unite to Remain group, which is set to unveil more details at a press conference on Thursday.
In a separate agreement earlier this year in the Brecon and Radnorshire by-election, the Lib Dems took the seat from the Tories after the other two parties stood aside.
The Liberal Democrats have vowed to put a £50 billion "Remain bonus" into public services if they win the general election on December 12.
The party said scrapping Brexit would generate billions of pounds for public finances.
Ms Swinson said every vote for the Lib Dems is a vote to "stop Brexit" and "build a brighter future".
A spokesman for the Greens earlier confirmed that the parties were "having discussions" but refused to say whether an electoral pact had been agreed.
In 2017, the party said 22 candidates were standing aside to boost the electoral hopes of opposing progressive candidates.
Under Theresa May, the government committed to reaching net-zero greenhouse gas emissions by 2050.
But the Green Party hopes to better that goal with its so-called Green New Deal, which it hopes will also encourage increased investment from the private sector.
The spending commitments come after a year of increasing pressure from environmentalists, including the direct action group Extinction Rebellion.
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Last month, Ms Berry's colleague and party co-leader Jonathan Bartley was arrested during an Extinction Rebellion protest on Whitehall.
A Green Party spokesman said at the time: "Climate chaos will end ecosystems and collapse our society. We have just 10 years to reduce our CO2 emissions to safer levels and climate protesters are drawing attention to that."
