The Liberal Democrats, Plaid Cymru and the Green Party have formed a Remain alliance, and will not run candidates against each other in a number of seats in the December 12 election.
The electoral pact will cover 61 constituencies across England and Wales.
The deal between the three pro-Remain parties was brokered by the Unite to Remain group, launched in August by former South Cambridgeshire MP Heidi Allen.
It follows an agreement made earlier this year in the Brecon and Radnorshire by-election, where the Liberal Democrats took the seat from the Conservatives after the other two parties stood aside.
Lib Dem leader Jo Swinson said the pact was a "significant moment" for voters opposed to Brexit.
"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," she said.
And Heidi Allen, chair of Unite to Remain, said the move was "unprecedented in modern British political history".
"With a single Remain candidate in each of these seats we can deliver a greater number of MPs into parliament. This is our opportunity to tip the balance of power away from the two largest parties and into a progressive Remain Alliance," she said.
Meanwhile, the Labour Party and Conservative Party are set to lock horns over the economy on the campaign trail later today.
In his first speech of the general election campaign, shadow chancellor John McDonnell will say that if Labour gain power on December 12, they will deliver a programme of investment "on a scale never seen before in this country".
His Conservative counterpart, chancellor Sajid Javid, will accuse Mr McDonnell and Jeremy Corbyn of behaving like "anti-vaxxers" on the economy.
He is expected to say that a Labour government would take "every step imaginable" to make the country "sick and unhealthy" again.
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