11 seats in Wales are part of the 'Remain Alliance' between the Liberal Democrats, Plaid Cymru and the Green Party.
Some of the seats affected include the Vale of Glamorgan, Cardiff Central, Ynys Môn and Caerphilly.
The agreement between the three parties will see them not stand against each other in more than a quarter of Welsh constituencies in the upcoming General Election.
The aim of the cross-party Unite to Remain group is to maximise the chance of pro-remain MPs winning seats on December 12.
The arrangement was announced at a joint press conference attended by representatives from the three parties.
As part of the alliance, the Green Party will put forward one candidate in the 11 constituencies whilst the Liberal Democrats will stand in three and Plaid Cymru in seven.
The seats affected and which party will stand:
Vale of Glamorgan - Green Party
Brecon and Radnorshire - Liberal Democrats
Cardiff Central - Liberal Democrats
Montgomeryshire - Liberal Democrats
Arfon - Plaid Cymru
Caerphilly - Plaid Cymru
Carmarthen East and Dinefwr - Plaid Cymru
Dwyfor Meirionnydd - Plaid Cymru
Llanelli - Plaid Cymru
Pontypridd - Plaid Cymru
Ynys Môn - Plaid Cymru
The Vale of Glamorgan - Green PartyThe Vale of Glamorgan was won by Conservative Alun Cairns in the 2017 General Election. Despite recently resigning from his ministerial role as Secretary of State for Wales, he has not stood down as a candidate for the Vale of Glamorgan at this election.
Brecon and Radnorshire - Liberal DemocratsLeader of the Welsh Liberal Democrats, Jane Dodds, won the Brecon and Radnorshire seat in a by-election in August 2019 and she will stand again this time around. She was responsible for reducing the Conservative Party's majority to one seat in the House of Commons. Plaid said they decided not to field a candidate in the by-election "to allow the pro-Remain vote to coalesce around Jane Dodds".
Cardiff Central - Liberal DemocratsThe Green Party and Plaid Cymru will step aside in Cardiff Central for the Liberal Democrats' candidate Bablin Molik. The seat has been won by Labour's Jo Stevens in the past two general elections but was won in 2005 by the Lib Dems.
Montgomeryshire - Liberal DemocratsThe Conservatives won this seat with over half of the vote share in the 2017 General election. The Liberal Democrats previously held the seat here four elections in a row between 1992 and 2005. However, since 2010 Conservative Glyn Davies has represented the constituency.
Arfon - Plaid CymruPlaid Cymru's Hywel Williams will stand here with the Liberal Democrats agreeing not to stand against him. He has been the MP in Arfon since 2010.
Caerphilly - Plaid CymruAlways electing a Labour MP, Wayne David has been the MP for Caerphilly since 2001. Plaid Cymru have usually come behind both the Conservatives and Labour at previous General Elections here.
Carmarthen East and Dinefwr - Plaid CymruPreviously held by Plaid's current leader Adam Price, this constituency has been represented by the party since 2001. The Green Party did not put forward a candidate here in the 2017 General Election.
Dwyfor Meirionnydd - Plaid CymruDwyfor Meirionnydd is typically a Plaid safe seat with Liz Saville Roberts winning in the past two General Elections. She is the party’s Westminster leader and will stand again as the candidate for Dwyfor Meirionnydd.
Llanelli - Plaid CymruNia Griffith has won the Llanelli seat for Labour in the past four General Elections. She won with more than half of the vote share in 2017 whilst Plaid secured just over 18%.
Pontypridd - Plaid CymruLabour's Owen Smith has won the seat in Pontypridd since 2010 however he announced in October 2019 that he will not stand in December's election. He said he made the decision because of "political and personal reasons".
Ynys Môn - Plaid CymruPlaid last won the seat in Ynys Môn in 1997, since then the area has been held by Labour's Albert Owen. Mr Owen has announced he will not be standing as Labour's candidate this year and is being replaced with Mary Roberts. The seat is a three-way marginal with Plaid and the Conservatives coming close to Labour's previous wins.
The leader of Plaid Cymru, Adam Price, said the move demonstrates that they are "putting the Welsh national interest first".
The arrangement that has been announced today between Plaid Cymru, the Liberal Democrats and the Greens represents the kind of grown-up politics that people expect. Plaid Cymru was the first party to seek cooperation between pro-Remain parties following May’s European election. I am pleased that, following months of discussions, we have successfully reached an arrangement.
The Liberal Democrat's leader in Wales, Jane Dodds, said her win in Brecon and Radnorshire demonstrates the previous success of cross-party alliances.
Plaid Cymru stepped aside in the 2019 by-election which Jane Dodds won.
During the by-election in Brecon and Radnorshire, we showed what can be achieved when parties put aside their differences and work together.
Meanwhile Jo Swinson, leader of the Liberal Democrats, has also welcomed the announcement saying she is "delighted" by the arrangement.
In response to the announcement, the Electoral Reform Society have criticised the current electoral system which they say has "forced" parties to form the alliance.
They say voters "lose out" because of the first-past-the-post system which leads to parties taking tactical decisions.
They are calling for a reform of the system and the introduction of proportional representation to give voters what they call "fair results".
Parties are feeling forced to game a voting system that fails to reflect the diversity of politics in 2019.