Labour deputy leader Tom Watson has hit back after Jeremy Corbyn accused him of talking "nonsense" about a hard-left infiltration of the party.
Corbyn said his deputy was utterly wrong to suggest that Labour's 300,000 new members were "sectarian extremists" – but Watson insisted he had never said all of them were.
It comes after Watson used an open letter on Thursday to warn that some Momentum activists were adopting the tactics used by the Militant group in the 1980s to take over party meetings.
Relations between Watson and Corbyn have hit a new low in recent weeks as the leader faces a challenge from Owen Smith in the wake of a no confidence vote from Labour MPs.
Corbyn told The Observer on Sunday that he had read Watson's letter in the media "and it appeared to be a rehash of a book Michael Crick wrote 20 years ago about alleged entryism into the Labour party at that stage".
He said: "I just ask Tom to do the maths: 300,000 people have joined the Labour party – at no stage in anyone’s most vivid imagination are there 300,000 sectarian extremists at large in the country who have suddenly descended on the Labour party.
"Sorry Tom, it is nonsense – and I think he knows it’s nonsense. Let’s get on with campaigning, Tom. Thanks."
In a statement released hours later, Watson said: "I know Jeremy is very busy on the campaign trail and is therefore relying on others to tell him what I have said.
"To repeat again, I have never said that all our new members are Trotskyists. I have never claimed that hundreds of thousands of new joiners are revolutionary socialists and those who claim I did are attacking a straw man."
He added: "I simply want to ensure that organisations like the Alliance for Workers' Liberty (AWL), who have instructed all their members to join the Labour party and target our youth sections for recruitment, are dealt with under our rules.
"It's undeniable that this is happening. AWL even published these instructions on their website."
On his Facebook page, Watson pointed to three documents posted on the AWL website that he said "outline a concerted strategy for members of the AWL to infiltrate (or, to use the language of the documents, 'intervene') in the Labour party with the explicit intention of influencing the party to indoctrinate 'more people of revolutionary socialist ideas'".