Plaid Cymru, Liberal Democrats and the Greens have agreed a pro-EU electoral pact in 11 of the 40 seats in Wales, BBC Wales has been told.
The deal means candidates will stand aside for each other in those 11 seats to increase the chances of a Remain-supporting MP being elected.
It follows a deal by the three parties in Brecon and Radnorshire’s by-election in August, won by the Lib Dems.
An official announcement will be made in central London on Thursday.
The plan is understood to involve the Liberal Democrats and Greens standing aside for Plaid Cymru candidates in three of the four seats Plaid is defending in the December election – Arfon, Dwyfor Meirionydd and Carmarthen East and Dinefwr.
Plaid Cymru is defending Arfon by a slender margin of just 92 votes.
The deal does not involve the Ceredigion seat – currently held by Plaid Cymru but a top election target for the Lib Dems who are also expected to have a free run at the Brecon and Radnorshire seat, won by their leader in Wales Jane Dodds in the August by-election.
A single Lib Dem candidate is also expected to contest the Montgomeryshire and Cardiff Central seats – two seats the party has held in the recent past.
In return, the Greens will be given a free run in at least one seat.
After May’s EU Parliamentary elections, Plaid Cymru leader Adam Price AM wrote to several pro-Remain parties, including the Lib Dems and Green Party, calling on them to work together in a snap general election or second EU referendum.
The discussions that led to this electoral pact stemmed from that letter.
On Sunday, Plaid Cymru’s Helen Mary Jones AM said she was “not entirely comfortable” with electoral pacts but they were the “right thing to do” for the 12 December general election.
Speaking on Wednesday, Liberal Democrat leader Jo Swinson claimed her party could win a significant number of seats in a “Remain alliance” without the need to do deals with Labour.
Launching its election campaign on Wednesday, Green Party co-leader Jonathan Bartley said he hoped the deal would lead to a big “Remain party bloc in the Parliament” but admitted some local party branches were against the idea of standing aside for a Lib Dem or Plaid Cymru candidate.
Prime Minister Boris Johnson last week rejected the idea of working with the Brexit Party during the election after Nigel Farage called on him to drop his new Brexit deal with the EU in a “Leave alliance”.
Labour leader Jeremy Corbyn has ruled out the idea of entering electoral pacts with other parties while First Minister Mark Drakeford said he was “instinctively not attracted” to the idea.