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.
The Liberal Democrats have joined forces with Plaid Cymru and the Green Party and agreed not to run candidates against each other in seats across the United Kingdom on 12 December.
The by-election victory prompted much speculation over whether a larger deal across Wales and England could be agreed between parties that support remain. The Greens will also be the only "remain candidate" in eight other seats in England and the Vale of Glamorgan in Wales.More news: Italy to include climate change on school curricula
The Unite to Remain group will unveil details of the seats covered by the new tie-up at a press conference on Thursday. The Greens will have a clear run at 9 seats, including Brighton Pavilion and the Isle of Wight.
"This historic cross-party initiative gives us the best chance to return more Remain MPs to further the fight to stop Brexit and I would like to thank Plaid Cymru and the Greens for taking part in these talks".
"The number of seats achieved, and the scope of what we're seeking to achieve, is unprecedented in modern British political history".
Labour is offering a further referendum on remaining in the European Union should it win the election and has dismissed the pact as the Lib Dems and Plaid "propping each other up" in seats they fear they could lose.More news: U.S. inmate seeks release from jail after he 'died'
'This arrangement will help elect more pro-Remain MPs in the next parliament, ' said Jo Swinson, leader of the Lib Dems, who have pledged to reverse Brexit. "Our brighter future is built by working together so we can Unite to Remain".
The Liberal Democrats, Plaid Cymru, and the Greens have agreed to step aside for each other in 60 seats across England and Wales in the general election.
Whereas the Liberal Democrats have said they will cancel Brexit, Labour's Jeremy Corbyn is adamant he wants to respect the result of the referendum but simply wants to renegotiate the deal to improve workers' rights and environmental protection.More news: Rep. Nunes Demands Adam Schiff Testify in Impeachment Inquiry