Labour deputy leader Tom Watson has that warned hard left supporters of Jeremy Corbyn are plotting to take control of the party amid claims of a secret deal with the Unite union boss Len McCluskey.
Mr Watson said it was clear the threat was real after a recording emerged of the founder of Momentum - the grassroots activists group which helped propel Mr Corbyn to the leadership - discussing the prospects of gaining Unite's backing.
Jon Lansman was said to have told supporters he expected the union - together with the Communications Workers Union (CWU) - to affiliate to Momentum if Mr McCluskey wins his battle for re-election as Unite general secretary.
The Observer, which obtained a tape of his remarks, said they were secretly recorded at a meeting of a newly-formed Momentum branch in Richmond, south west London, earlier this month.
Mr Lansman is quoted as saying: "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."
He went on to tell activists it was "absolutely crucial" that they secured a change to the party's rules to ensure that whenever Mr Corbyn stood down, they were able to get a candidate on to the ballot paper to succeed him.
"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," he said.
Currently, a candidate must obtain the support of 15% of Labour MPs and MEPs in order to stand - a threshold a new left wing contender is unlikely to be able to meet.
Mr Watson said Mr Lansman's comments made clear the threat of "entryism" into the Labour ranks by hard left activists was genuine.
He said it was "a matter of great concern" that the Unite leadership was allegedly making "a private agreement to fund a political faction that is apparently planning to take control of the Labour party".
"I warned last year of entryism and no one can now doubt that threat is a real one," he 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 spokesman for Unite spokesman told The Observer that the general secretary could not make such a decision alone.
"Affiliation to Momentum is a matter for our executive council alone and no discussion on the matter is scheduled."
Momentum insisted there were no current plans for Unite to affiliate and that it had no received any funds from them.
A spokesman 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."
However, Gerald Coyne, who is standing against Mr McCluskey as general secretary, said: "I have been warning for weeks that Len was trying to act as puppet master to Jeremy Corbyn. Now the evidence is clear for all to see.
"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 the Union."