A hard-left plot by supporters of Jeremy Corbyn to seize permanent control of the Labour party and consolidate their power by formally joining forces with the super-union Unite can be revealed by the Observer.

The plans, described on Saturday by Labour’s deputy leader Tom Watson as “entryism” and a covert attempt by a leftwing faction to take over the party, were spelled out in detail by Jon Lansman, the founder of the grassroots organisation Momentum, who was secretly recorded addressing supporters at a meeting of a new branch of the organisation in Richmond, south London, on 1 March.

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On the tape, obtained by the Observer, Lansman issues a call to arms to Momentum supporters, saying they need to make sure the left is far better represented in key positions at all levels of the party so they have control over the levers of power when Corbyn departs and the succession is decided.

Most controversially, Lansman says that if his ally Len McCluskey secures re-election as general secretary of Unite in an internal election next month, the super-union will then link directly to Momentum by formally affiliating to it, in what critics fear would amount to a massive shift of power and financial resources to the pro-Corbyn left.

Announcing what he implies is a done deal with McCluskey, Lansman tells the audience: “Assuming that Len McCluskey wins the general secretaryship, which I think he will, Unite will affiliate to Momentum and will fully participate in Momentum, as will the CWU [the Communications Workers’ Union].”

The extent to which the left is mobilising behind the scenes and looking to Unite to back it at national and constituency levels will greatly alarm Labour moderates. Lansman spells out how Momentum currently lacks money. His mention of a link-up with Unite will invite inevitable speculation that the country’s biggest union – and Labour’s largest donor – is preparing to give money, as well as organisational support, to Momentum, too.

News of the plans will also alarm the many Labour MPs who sought to oust Corbyn in a coup last summer, and who now worry that leftwing activists and some Unite insiders are laying plans to deselect them in a mass purge before the next election.

Watson, a Unite member, voiced his deep concern about what he said looked like “a private agreement to fund a political faction that is apparently planning to take control of the Labour party, as well as organise in the GMB and Unison”.

He said it was evidence of the kind of “entryism” he had warned about. “I warned last year of entryism and no one can now doubt that threat is a real one,” Watson said. “For Unite to affiliate to Momentum it would require the approval of its executive committee. I hope Len McCluskey hasn’t made promises without clearing them through the democratic structures of our union.”

A spokesperson for Momentum said: “Jon Lansman was speaking in an aspirational manner about the possibility of Unite and the CWU affiliating to Momentum and stating that if they did choose to do so they would, like other affiliated unions, be able to take part in Momentum’s affairs by having a seat on its national co-ordinating group. Momentum’s constitution allows for up to six places for trade union affiliates, which have been taken up by some unions already.” Momentum sources said Unite had not given it any funds up to now and that there were no current plans for Unite to affiliate to Momentum.

A Unite spokesman said affiliation decisions could not be made by the general secretary alone: “Affiliation to Momentum is a matter for our executive council alone and no discussion on the matter is scheduled.”

At the meeting, Lansman suggested that one urgent task was to boost the left’s representation in key positions – particularly on the national executive committee (NEC) – so it could secure rule changes that would help install a leftwing successor to Corbyn.

Facebook Twitter Pinterest Len McCluskey’s Unite union is Labour’s main financial backer. Photograph: Neil Hall/Reuters

In the recording, Lansman talks openly about Corbyn’s future, saying that while he hopes he will continue as leader, preparations must be made for the moment he goes. He says responsibility for the party’s disastrous performance in the Copeland byelection last month, where it suffered a crushing loss to the Tories in a previously solid Labour seat, lies with the man at the top.

While Lansman says he does not believe any other leader would have done better, he adds: “That is not an excuse. Jeremy is leader of the Labour party and we, as Momentum, are a really important influence in the party’s direction and we have to find a way forward.” He goes on: “Ensuring that when Jeremy ceases to be leader, and at some point he will cease to be leader, I hope at a time of his own choosing, we have a fair election where candidates who have support among the membership can get on the ballot paper and we will be able to vote for them.”

It is, he says, “absolutely crucial” that the left is strong enough to ensure there is a change to party rules that currently require candidates for the leadership – in the event of a vacancy – to secure backing from at least 15% of MPs and MEPs. A hard-left candidate is thought unlikely to be able to reach the 15% threshold. But if it can be lowered, then the left’s choice will be in a far stronger position to get on the ballot and follow in Corbyn’s footsteps by winning the vote of the mass membership.

Last night, Gerard Coyne, the Unite official who is challenging McCluskey, said there appeared to be a plot to use union members’ cash to fund the hard left. “This shocking revelation reveals a secret hard-left plot by Len McCluskey to seize control of the Labour party in perpetuity using cash taken from hard-working members of Unite,” Coyne said.

“As far as Len McCluskey is concerned, Unite’s members’ money is his to play with as he chooses, from taking out a loan to allow him to buy a luxury London apartment to propping up the ultra left. This is not what trade unions are for. It is time for a change.”

Lansman says on the tape that Corbyn and John McDonnell, the shadow chancellor, personally asked him to exclude members of the Socialist party (the successor to Militant Tendency) from Momentum owing to the embarrassment they were causing. But he made clear to the audience that, despite changing Momentum’s rules so that only people who were Labour members could join, this would not be enforced in such a way as to exclude people. “There is no one to my knowledge in Momentum – certainly not me – who wishes to exclude people. It was important to require Labour party membership in the rules, but it is down to enforcement. No one from the centre is going to tell you to kick people out.”

The Momentum spokesperson explained that its previous steering committee voted to introduce its new constitution to deliver the campaigning movement its members wanted. “As such, new members of Momentum must be members of Labour to join Momentum, and existing members of Momentum have the opportunity to join the party by 1 July.”