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 three anti-Brexit parties will announce details of the deal on Thursday, but it is thought to cover between 60 and 70 constituencies.
In Wales, the pact will cover 11 of its 40 seats, BBC Wales has been told.
Such a pact means two of the parties would agree not to field a candidate, boosting the third candidate’s chances.
Outside of Wales, the pact 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.
All three parties have already launched their election campaigns. They all support another Brexit referendum and want to Remain in the EU.
Last month, it emerged the trio were in negotiations about pacts in certain constituencies to ensure a pro-Remain candidate is elected.
“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 new pact followed a similar deal earlier this year in the Brecon and Radnorshire by-election, when Plaid Cymru and the Greens agreed to put forward a candidate but instead give way to the Lib Dems’ Jane Dodds.
Ms Dodds went on to defeat the Conservative incumbent, Chris Davies.
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 is understood to involve the Lib Dems and Greens standing their candidates aside for Plaid Cymru in three out of the four seats that Plaid is defending: Arfon, Dwyfor Meirionydd and Carmarthen East and Dinefwr.
In Arfon in 2017, Plaid Cymru won by just 92 votes.
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.
Read more about the pact in Wales
Last week, the Brexit Party’s Nigel Farage called on Boris Johnson to form a similar election pact. Mr Farage 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 into election pacts.
Labour leader Jeremy Corbyn has also ruled out the idea of entering electoral pacts with rival parties.