An anti-Brexit campaign group says dozens of MPs have backed its plan to force a second referendum that could stop Britain leaving the EU.

Best for Britain said it had secured the backing of around 40 Labour, Liberal Democrat and Green Party MPs for a “people’s vote”.

The group is calling on the government to give the public the final say on the Brexit deal Theresa May hopes to negotiate with the EU.

It has raised £2.3m for the project, including from billionaire businessman George Soros - although only a fifth of the group's funding comes from him.

Best for Britain said a public vote would settle the debate about Britain’s relationship with the EU for good, claiming “uncertainty is as great a threat to the UK as Brexit itself”.

In an open letter to voters, it said: “After 29 March 2019, our option to stay in will disappear. It is only right that the people get to decide whether to forfeit our current deal after having the opportunity to compare it to the Brexit deal. It’s simple: may the best deal win.

“We believe the Brexit process is distracting from the many reasons people voted to leave: a kick back against Westminster to wake up to the reality of life in modern Britain – an industrial wasteland in parts of our country and the loss of good jobs, a weakening health service, unaffordable housing, and rising student debt."

Attacking both main political parties, it said: “This is the most important national decision for decades, and the government’s ship is rudderless. On the opposition benches, Labour is dithering.”

The group's plan would see MPs table an amendment to the government’s Withdrawal Agreement and Implementation Bill, due to be published later this year.

If backed by Parliament, a referendum on the deal would then be held before 29 March 2019 – the day Britain is due to leave the EU.

Best for Britain is launching a nationwide campaign to convince the public to back calls for a second referendum. It will hold events in around 70 marginal constituencies and spend £500,000 on billboards and newspaper adverts in key areas.

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Lord Malloch Brown, a former Labour Foreign Office minister and the chairman of Best for Britain, told The Independent: “If we let Brexit go on its current slow unfolding of Tory cabinet disputes followed by confusion in Parliament, and somehow this thing limping through to some kind of headline agreement only by October, then that indecision is going to last for years.

"This process of parliament [approving] a people's vote is a response to the complete absence and abdication of party political leadership on both sides. The most important decision ever and the government can't get its act together to tell us what it's proposing, the opposition can't get its act together to interrogate that government and expose that shortcoming,"

David Davis wants post-Brexit relationship with Europe that 'recognises the history' and 'stands the test of time'

Discussing the potential for a "people's vote", he said: "We will campaign robustly and vigorously for Remain. We think in a campaign which is less about the emotional, lurid issues of the first one, but is now about weighing the real costs and benefits of leaving, that in that context we have a very good shot of winning."

He admitted it was a "major risk" to push for a second vote while there is no clear evidence the public wants to reverse Brexit, but said it was time to end the "ridifools gculous chasing of fool's gold". Losing the referendum and Brexit going ahead would be preferable to "a war of attrition that goes on for years", he added.

Best for Britain said its research suggests public opinion is moving against Brexit, and that it would be confident of securing an anti-Brexit majority in a second poll.

Launching the campaign in Westminster, Eloise Todd, Best For Britain's chief executive, said: "For too long we've been asked to swallow the lie that the votes of 17 million people - with their individual histories, experiences, and ideas - gave May a clear mandate to deliver whatever Brexit she can fashion, no matter how different that is from its original conception or how damaging it might prove to be.