Pro-EU political parties are set to take a major step towards forming a national “Remain alliance” by working together with a new organisation designed to increase the number of anti-Brexit MPs.

Independent MP Heidi Allen, who quit the Conservative Party earlier this year, will launch the “Unite to Remain” group on Wednesday.

The organisation will work with the Liberal Democrats, Green Party, Plaid Cymru, Change UK and other anti-Brexit parties to ensure that the pro-EU vote is not split between them in key elections.

It will conduct a seat-by-seat analysis and encourage the parties to agree to step aside where they have little chance of victory and instead support whichever of them has the best chance of winning.

The move comes amid a growing belief in Westminster that a general election within the next year is highly likely, and follows speculation that a Conservative Party led by Boris Johnson could form an electoral pact with Nigel Farage's Brexit Party.

Ms Allen said all of the Remain parties had agreed to be involved with the new organisation. They have already agreed to field only one candidate in next month’s Brecon and Radnorshire by-election, and Unite for Remain is designed to encourage a similar approach across the country.

Ms Allen will chair the group’s advisory council, which will consist of the party leaders and senior figures from the Remain parties. Insiders said it was important that the initiative was led by an independent MP who did not have an agenda in favour of any particular party.

Unite for Remain is aiming to form an alliance in between 150 and 180 seats across the country, although the unwillingness of the SNP to take part means it is unlikely to focus its efforts on Scottish constituencies.

Donors have also been lined up to help support candidates who stand under the Unite to Remain banner.

Speaking to The Independent, Ms Allen said voters were “crying out” for parties to work together, and that the “high likelihood” of a general election required “urgent” action to boost the number of Remain-supporting MPs.

She said: “Brexit has shifted the tectonic plates and shifted politics and our way of operating more than anything before. With both Labour and the Tories moving to the extremes, people just don’t recognise politics any more.

“The only way we’re going to break out of that under first-past-the-post is to learn to work differently. I’m genuinely encouraged by the appetite in all the parties to try. We might not succeed and get as many seats as we’d like but we need to try to build a genuine centre-ground, internationalist movement."

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Insisting that there was significant support for a new alliance, she added: “The risk of Boris Johnson and Nigel Farage twinkling over the horizon has really focused people’s minds. It is unprecedented. People in party politics don’t typically behave like this but I’ve been pleasantly surprised by the appetite.”

Ms Allen said Unite for Remain would prioritise “modern, country-first, cross-party collaboration across the nation” and added: “Our country is crying out for mature and progressive politics, not a government elected to pursue old ideology from the left or right. We need to work together to increase the number of MPs who can bring this aspiration to life.”

Insiders said the group was formed partly in anticipation of the number of independent MPs increasing if there are further defections from the Conservatives once Boris Johnson becomes leader and from Labour if the party pushes ahead with deselections.