The Liberal Democrats, Plaid Cymru and the Green Party have formed a General Election pact, agreeing not to stand against each other in dozens of seats.
The deal, brokered by the Unite to Remain group, will give voters a single Remain choice in 60 constituencies across England and Wales.
The group is confident that “at least 44” of the 60 seats are “highly winnable”.
The move follows an agreement earlier this year in the Brecon and Radnorshire by-election, where the Lib Dems took the seat from the Conservatives after the other two parties stood aside.
Heidi Allen, chairwoman of Unite To Remain and previously MP for South Cambridgeshire, said the cross-party arrangement is “unprecedented in modern British political history”.
In total, the Lib Dems will stand in 43 constituencies, the Greens will stand in 10 and Plaid Cymru will stand in seven.
In the Isle of Wight and Brighton Pavilion, the Greens will be given a free run, while in the constituencies of Richmond Park, Guildford and Cheltenham, the Lib Dems will be unopposed by other Remain-backing parties.
Lib Dem leader Jo Swinson said she is “delighted” about the agreement, adding that it is a “significant moment for all people who want to support Remain candidates across the country”.
Ms Allen, who is not standing in the election, told the PA news agency: “I thought when I got into Parliament as an MP that not on everything, but on big issues that would really affect the country’s future Parliament would work together, that the Government would reach out to the opposition.
“And they just haven’t done that at all.
“We’ve had two massive failures of leadership from the Conservative Party and from Labour.”
I am delighted that we’ve been able to announce this arrangement to make sure we get as many pro-Remain MPs as possible. Our brighter future is built by working together so we can #UniteToRemain https://t.co/NiYbxMM9vd
— Jo Swinson (@joswinson) November 7, 2019
Ms Allen said the Lib Dems, the Greens and Plaid are saying: “You know what? Enough. Somebody has to step up and offer proper leadership on Brexit.”
She said there are three core issues which the parties share views on – a second EU referendum, proportional representation and climate change.
Ms Allen added: “Secretly, I’m quite excited. I hope that if this is successful, they’ll look back and go, ‘Do you know what guys? That worked pretty well. Shall we try that again?’
“Because we look at European politics, coalition-based progressive alliances.
“I’d love to see that here too.”
At a press conference in London, Peter Dunphy, election strategist and a director at Unite to Remain, said: “At least 44 of those 60 can be regarded as highly winnable constituencies.”
He said this evaluation is based on a wide range of data, including current national opinion polls, local election results and European elections.
“These will be seats in which the parties will be putting huge resource because they believe that they can win those seats,” Mr Dunphy said.
Ms Allen told gathered media: “As with so much in life, you’re stronger when you work together as a team.”
On the issue of the Labour Party not being part of the pact, Lib Dem president Sal Brinton said: “First of all, Jeremy Corbyn stands for Brexit. This is about Remain. That’s a fundamental block.”
Ms Brinton added: “The second thing is Labour have made it plain that under their rules, they will never stand back for any other party.
“And that I’m afraid is part of the old two-party style of politics.”
Ms Brinton said that with their pact, the parties are demonstrating that there is a “new way” of doing politics.
Plaid Cymru’s Liz Saville Roberts said she hopes the parties coming together gives “some credibility” to politics.
“Our message is clear. This is what matters to us and this is why we’re acting this way,” she told the press conference.
Green MEP Molly Scott Cato said she hopes it will be seen as a “historic day”, and that the deal will “break the two-party stranglehold”.
But Mike Powell, who was intending to stand in Pontypridd as a Lib Dem candidate, said Lib Dems in Wales are “extremely unhappy” with the way the negotiations were dealt with.
Layla Moran, Lib Dem candidate for Oxford West & Abingdon, said it is “really regrettable” that Mr Powell is standing as an independent, telling the World At One: “I think for the vast majority of candidates it is with agreement, and some of the ones I’ve spoken to have actually done it almost enthusiastically because they can see that bigger picture of how working together puts across that strong message of actually grown-up politics, and fighting first-past-the-post, and we all know that the electoral system is not our friend in these situations.”
Ms Moran said the Labour Party was approached by those who set up Unite to Remain and asked if they would like to be included, but said no.
“We know that Labour aren’t a Remain party, so perhaps that’s why,” she said.
The Unite to Remain pact comes as similar electoral manoeuvres are taking place in Northern Ireland.
The Green Party NI announced it will not stand in all four Belfast constituencies to help other pro-Remain parties, while Sinn Fein and the SDLP are both stepping aside in three constituencies to help efforts to unseat DUP candidates.