Britain's Liberal Democrats, Plaid Cymru and the Green Party announced a pro-Remain electoral pact on Thursday by which they will not field candidates against each other for parliamentary seats across England and Wales.

The three anti-Brexit opposition parties are targeting 60 seats.

"Each party has their own priorities. Each proudly stands up for what they believe in. But each is also subject to one very important truth. None of them are more important then what's best for this country," Heidi Allen, chair of the Unite to Remain alliance, said in a video.

Allen was elected an MP in 2015 for the ruling Conservative party but defected in early 2019 over its Brexit strategy and later joined the ranks of the Liberal Democrats. She has announced, however, that she would not seek reelection citing Brexit but also bullying as reasons.

The pact, which does not include the main opposition Labour party, is expected to hold for some 60 of the 650 seats up for grabs in the House of Commons.

READ MORE: Why tactical voting is the talk of the UK's upcoming general election

The alliance is favouring the Liberal Democrats given that they currently have the strongest delegation in Westminster with 20 MPs to Plaid Cymru's four — the party is only implanted in Wales — and the Greens' sole parliamentarian.

Of the 60 constituencies in play, the Lib Dems will stand down in just 17 while Plaid Cymru and the Greens will do so in 43 seats.

"We are delighted that an agreement has been reached," Lib Dem leader Jo Swinson said in a statement.

"This is a significant moment for all the people who want to support remain candidates across the country," she added.

Adam Price, leader of Plaid Cymru, said that "in these deeply serious times we need grown-up politics that puts our countries before parties. The single most important thing in this election is that we return as many pro-remain MPs back into Parliament as possible."

The pact comes months after the three parties already successfully worked together to see a pro-Remain candidate elected in a by-election in Wales' Brecon and Radnorshire constituency.

Plaid Cymru and the Green party agreed not to field their own candidates to increase the Liberal Democrats' chances at defeating the incumbent Conservative MP.

READ MORE: Lib Dems win Welsh by-election, cut Johnson parliament majority to one