A bitter power struggle within the People’s Vote campaign between the PR guru Roland Rudd and the Labour peer Peter Mandelson has sent the alliance into crisis before a crucial week when MPs will finally test support for a second referendum in parliament.

The organisation is riven by a battle over who will have control over a remain campaign in any future referendum, even though the case for another vote on Britain’s EU membership has not yet been won.

People’s Vote said hundreds of thousands of campaigners turned out to a rally in central London on Saturday to oppose Boris Johnson’s Brexit deal, and supportive MPs will attempt to amend legislation to force a second referendum this week.

Labour could back Brexit bill if second referendum attached, says Starmer Read more

But behind the scenes, there are ongoing rows centring on Rudd’s moves to start a future remain campaign, led by a new female chair, bringing together large numbers of pro-EU bodies and removing some of the current leadership.

Rudd told grassroots activists last week that he would step down as chair of Open Britain, the most powerful organisation under the People’s Vote umbrella, while pushing for a wider shake-up and restructuring of the campaign’s leadership.

As part of that, there are moves to oust James McGrory, the director of People’s Vote, and Tom Baldwin, its director of communications. About 60 people work for People’s Vote at offices in Millbank, but the shape of the campaign in its current form is now uncertain.

One senior supporter said there had been a “clash of egos and personality” between Rudd, the brother of the former Tory cabinet minister Amber Rudd, and Mandelson. But they said there were also disagreements over how “remainy” the campaign should be and how far it should be linked to a wider realignment of politics, rather than just a vehicle through which to push for a second referendum.

Private emails leaked to the Mail on Sunday suggested Mandelson and Alastair Campbell, another former New Labour communications adviser, had been attempting to work together against Rudd this year. In one email, Campbell said: “I do not see how this gets done without a public battle and it should happen soon and be fast and brutal.”

Another People’s Vote insider said the dispute was about Mandelson and Campbell taking a stand against Rudd, who they accuse of trying to take control of the campaign and get rid of senior staff.

It comes at a time when MPs linked to the campaign are trying to win over soft-Brexit Tories, such as Oliver Letwin, Philip Hammond and David Gauke, to their cause in parliament, without scaring them off by turning into the pro-remain campaign of a referendum.

People’s Vote MPs were shown a presentation by the campaign in recent weeks, known as Project Marshall, suggesting that any remain campaign would need to target 60% of the vote share by changing the minds of former leave supporters, and not just appeal to people who are already passionately pro-EU.

A People’s Vote spokesman said: “The People’s Vote campaign this weekend organised one of the biggest marches in British history [and] a letter-writing campaign that has attracted 150,000 signatories in just a few hours, while we will carry on working closely with MPs standing up against Boris Johnson’s imposition of a hard Brexit in these crucial days ahead. All of us on this campaign, frankly, have better things to do than worry about who chairs the board of Open Britain – which is just one of the organisations that make up our brilliant campaign.”