Jeremy Corbyn’s biggest union backer has publicly condemned the party’s deputy leader over comments he made regarding Momentum’s apparent plan to take control of Labour and affiliate to the union Unite.

Dismissing the affiliation claims as “entirely inaccurate”, Britain’s biggest union also accused Tom Watson of a “misguided” campaign to interfere in Unite’s internal democracy and its ongoing general secretary election.

It came after Mr Watson pleaded with the Labour leader to “deal with” Momentum and its founder Jon Lansman in a series of radio and television interviews on Monday. He said it was clear the party’s future was at risk after a recording emerged of Mr Lansman discussing the prospects of gaining Unite’s backing.

Shortly after a meeting of Labour’s Shadow Cabinet and in an attempt to show a united front, Mr Watson and Mr Corbyn put out a joint statement, agreeing on the urgency to strengthen party unity.

The Labour leader and deputy leader added: “The Shadow Cabinet met today to discuss Labour’s policy and election plans and had a robust and constructive discussion about the challenges and opportunities ahead.

“It recognised the right of groups across the spectrum of Labour’s broad church to discuss their views and try to influence the party so long as they operate within the rules.”

But in an extraordinary statement Unite’s acting general secretary Gail Cartmail had said earlier: “Tom Watson has made claims about Unite and its general secretary Len McCluskey which are entirely inaccurate. As Unite has made it clear it is exclusively for our executive council to determine which organisations we affiliate to.”

Ms Cartmail added there were no plans for Britain’s biggest union to affiliate to Momentum – the grassroots organisation of left-wing activists, set up in the wake of Mr Corbyn’s election as Labour leader

She added that Len McCluskey, who is currently campaigning to retain his position as Unite’s general secretary, has never met Mr Lansman to discuss affiliation or any other matter.

“It is extraordinary that the deputy leader of the Labour Party should interfere in Unite’s democracy this way and it is very disappointing he was allowed to make his unsupported claims without being challenged, and that the BBC ignored the Unite statement which it had been provided well in advance,” she said.

“Mr Watson’s latest, and misguided, campaign is part of an unprecedented pattern of interference in the current Unite general secretary election by elected Labour politicians who should, frankly, be concentrating on their own responsibilities.

“Mr Watson is a Unite member with a right to a vote and a view. But he should remember that, first, he is deputy leader of the Labour party with the obligations that this senior post imposes, and second that Unite is not a subsidiary of any political organisation.”

The bitter recriminations between the union and Mr Watson come after the weekend leaking of a recording of Jon Lansman, Momentum’s founder, addressing supporters at a meeting in Richmond, South London, on March 1. On the tape, obtained by the Observer, Mr Lansman said Unite would formally affiliate with Momentum, if Mr McCluskey secures re-election later this month.

Momentum looks like it is orchestrating a 'takeover from the hard left' says Tom Watson

The plan appeared to be for the country's biggest union - and Labour's largest donor – to channel its huge funds, as well as organisational support, to the grassroots group.

Mr Lansman also called for a “a fair election” when Mr Corbyn steps down, backing a cut in the current requirement for leadership candidates to have the backing of at least 15 per cent of MPs and MEPs.

Speaking on BBC’s Radio 4’s Today programme, Mr Watson said Momentum’s founder had “outlined a hard left plan to control the Labour party after Jeremy’s departure”.