Len McCluskey has dismissed as "ridiculous" claims his Unite union is plotting a hard-left takeover of the Labour Party.
He also poured scorn on deputy Labour leader Tom Watson's warning that supporters of Jeremy Corbyn were involved in a "secret plot" to destroy the party, saying it was "a complete fabrication".
He added: "Tom and the other right-wing Labour MPs would be best keeping their nose out of our business.
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"The truth of the matter is, my members will reject any attempt from outside bodies to influence and to try to take over Unite."
McCluskey's denials come after Jon Lansman, founder of the Corbyn support group Momentum, was secretly recorded by The Observer saying that if McCluskey were re-elected as Unite general secretary, the union would affiliate to Momentum rather than to Labour.
This would give the group access to funding and institutional support and represent a major realignment in the balance of power on the left.
Watson yesterday called on Corbyn to "deal with" Momentum, which he accuses of colluding with Unite to gain control of the party.
Speaking on BBC Radio 4's Today programme yesterday, he said it was his duty to "call out" what appeared to be a "secret plan" to take over Labour by "people who do not have our electoral interests at heart".
He added: "I regard this as a battle for the future existence of the Labour Party."
Shadow chancellor and Corbyn ally John McDonnell accused Watson of "interfering" in Unite's leadership contest.
The Daily Telegraph says Labour has now descended into a "state of war". The New Statesman quotes a Momentum spokesperson claiming Lansman's comments were merely "aspirational", while Unite has denied it has any formal plans to affiliate to the group.
However, friends of Watson believe there is an "orchestrated attempt" by the two organisations to "take control of the machinery of the Labour Party", BBC assistant political editor Norman Smith says.
According to Smith's sources, Momentum is pushing for mandatory reselection of MPs, the removal of critical councillors and the takeover of regional Labour parties.
Watson was "cheered to the rafters" at a meeting of Labour MPs last night, the BBC reports.
Lansman was also recorded saying it was "absolutely crucial" that activists secured a change to the party's rules to ensure that when Corbyn stands down, they are able to get a left-wing candidate to succeed him.
Under existing rules, a prospective candidate must win the support of 15 per cent of Labour MPs and MEPs in order to stand, "a threshold a new left-wing contender is unlikely to be able to meet", says the BBC.
A change in the party's charter - dubbed the "McDonnell amendment" - to reduce that threshold to five per cent will be debated at the conference in September. A stronger Momentum, backed by Unite, could prove decisive.
This is the issue – far more than Trident, or nationalisation, or tax, or trade union legislation – that will "decide the fate of the Labour Party", says the Daily Telegraph.
Only Watson stands between the hard left and this act of electoral suicide, it adds.
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