Plaid Cymru, Liberal Democrats and the Greens have agreed a pro-EU electoral pact in 11 of the 40 seats in Wales.
Candidates will stand aside for each other in those constituencies to increase the chances of a Remain-supporting MP being elected.
Plaid Cymru will be given a free run in seven seats, the Lib Dems in three, and the Greens one.
The move is not universally popular, with a Welsh Lib Dem candidate and a campaigns manager resigning.
It follows a deal by the three parties over the Brecon and Radnorshire by-election in August, which saw the Lib Dems take the seat from the Conservatives.
The Lib Dems and Greens have also agreed to stand down for each other in 49 seats across England.
The plan involves 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 Meirionnydd and Carmarthen East and Dinefwr.
Plaid Cymru is defending Arfon by a slender margin of just 92 votes.
The deal does not involve Ceredigion - currently held by Plaid Cymru but a top election target for the Lib Dems who have previously held the seat.
The Lib Dems will have another free run in Brecon and Radnorshire, won by their leader in Wales Jane Dodds in the August by-election.
They will also contest Montgomeryshire and Cardiff Central, both previously held by the Lib Dems.
In return, the Greens will be given a free run in the Vale of Glamorgan.
Are people within the parties all happy?
Liberal Democrat parliamentary candidate for Pontypridd Mike Powell said he had resigned from the party in protest at the pact and would instead run as an independent.
He told BBC Radio Wales' Breakfast with Claire Summers: "I've never been against forming electoral pacts but there are red lines in certain instances and unfortunately for me the red line was crossed when the Welsh Liberal Democrats or the negotiators in London for the Welsh Liberal Democrats decided to stand us down in Pontypridd.
"I've campaigned in Pontypridd as a Lib Dem for 25 years.
"Unfortunately I put my resignation into the Welsh party today because I believe there are time when you have to give people the opportunity to vote for the person or the party that best meets their needs and wants."
He said the party had a "particularly weak leadership in Wales at this moment in time", adding: "That leadership has been unable to stand up for the Welsh Liberal Democrat Party against the federal party in London.
"It's a great shame and it's caused a great deal of anger and upset within the party."
Other candidates standing in Pontypridd include Alex Davies-Jones for Labour and Fflur Elin for Plaid.
The close of nominations is 14 November.
Karen Roberts, the Lib Dem campaign manager in Rhondda Cynon Taff, said on Twitter she was also resigning from the party with "huge regret", adding it was a "very sad day".
Where in Wales will the three parties support a single candidate?
- Arfon - Plaid Cymru (defending the seat)
- Brecon and Radnorshire - Lib Dems (defending the seat)
- Caerphilly - Plaid Cymru (held by Labour)
- Cardiff Central - Lib Dems (held by Labour)
- Carmarthen East and Dinefwr - Plaid Cymru (defending the seat)
- Dwyfor Meirionnydd - Plaid Cymru (defending the seat)
- Llanelli - Plaid Cymru (held by Labour)
- Montgomeryshire - Lib Dems (held by Conservatives)
- Pontypridd - Plaid Cymru (held by Labour)
- Vale of Glamorgan - Green Party (held by Conservatives)
- Ynys Mon - Plaid Cymru (held by Labour)
What do the other parties think?
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 accepted 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 Welsh Labour's First Minister Mark Drakeford said he was "instinctively not attracted" to the idea.
A Welsh Labour spokesperson said: "This pact appears to be nothing more than Plaid and the Lib Dems propping each other up in seats they're worried about losing. Plaid are worried about Arfon. The Lib Dems don't think they'll hold Brecon & Radnorshire.
"Both will fight it out over Ceredigion because it's not really an alliance, it's two parties looking out for their own self interest.
"If people want the final say on Brexit they should vote Welsh Labour, and in that public vote we will campaign to Remain."