And Adam Price, the leader of Plaid Cymru, said the strategy was "grown-up politics". It is significant because it could prove to be a decisive factor in marginal seats where Remain voters could split their vote between different candidates, and because previous cross-party attempts to co-ordinate Remain pacts have failed.
The parties "know this general election is the last chance to change the path that this country is now on".
"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".
Details of the arrangements, covering 60 constituencies, by the three pro-Remain parties will be announced at a press conference today by the Unite To Remain group, which brokered the deal.
Plaid Cymru, Liberal Democrats and the Greens have agreed a pro-EU electoral pact in 11 of the 40 seats in Wales.More news: United Kingdom police name Vietnamese truck victims
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In the Isle of Wight and Brighton Pavilion, the Greens will be given a free run, while in the constituencies of Richmond Park and Cheltenham, the Lib Dems will be unopposed by other Remain-backing parties.
'The number of seats achieved, and the scope of what we're seeking to achieve, is unprecedented in modern British political history. "This is our opportunity to tip the balance of power away from the two largest parties and into the progressive remain alliance".
However, the alliance does not include the Labour Party or the Scottish National Party, which will blunt its effectiveness because they now have the second and third-most MPs and support another Brexit referendum.
"If people want the final say on Brexit they should vote Welsh Labour, and in that public vote we will campaign to Remain".
Jo Swinson, the Liberal Democrats leader, said: "We are delighted that an agreement has been reached".
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.