The Liberal Democrats, Plaid Cymru and the Green Party have formed an electoral pact, agreeing not to stand against each other in dozens of seats.
The deal between the three anti-Brexit parties will cover 60 constituencies across England and Wales.
Chair of the Unite to Remain group Heidi Allen said it was “an opportunity to tip the balance of power”.
The three parties all support another Brexit referendum and want the UK to remain in the EU.
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.
Thursday marks exactly five weeks until the UK general election on 12 December.
“We are delighted that an agreement has been reached,” said Lib Dem leader Jo Swinson. “This is a significant moment for all people who want to support Remain candidates across the country.”
The pact follows a similar deal earlier this year in the Brecon and Radnorshire by-election, when Plaid Cymru and the Greens agreed not to put forward a candidate but instead gave way to the Lib Dems’ Jane Dodds. She went on to defeat the Conservative incumbent, Chris Davies.
In England, the Greens will stand aside for the Lib Dems in 39 seats including Totnes, York Outer, Winchester and Twickenham.
And the Green Party will run unchallenged by the Lib Dems in nine seats including the Isle of Wight, Bristol West, Exeter and Brighton Pavilion – where Caroline Lucas is the Greens’ only MP.
The pact comes after Plaid Cymru’s leader Adam Price wrote to several pro-Remain parties earlier this year, calling on them to work together in a snap general election.
In Wales, the plan will involve the Lib Dems and Greens standing their candidates aside for Plaid Cymru in seven seats including Arfon, which Plaid Cymru won by just 92 votes in 2017.
The deal does not involve the Ceredigion seat – which is currently held by Plaid Cymru but is a top election target for the Lib Dems.
Last week, Brexit Party leader Nigel Farage called on Boris Johnson to form a similar election pact. He wanted the PM to drop his Brexit deal and then agree to stand aside candidates for each other.
Mr Johnson rejected the offer and said he would not enter election pacts.
Labour leader Jeremy Corbyn has also ruled out the idea of forming electoral pacts with rival parties.