Jeremy Corbyn has directly attacked his elected deputy Tom Watson over his claims that "Trotsky entryists" are manipulating young party members to boost support for the Labour leader.
The pair's relatively peaceful public relationship erupted into a war of words last week with allies of Mr Corbyn accusing Mr Watson of peddling "baseless conspiracy theories".
The deputy leader replied with a dossier which he claimed backed up his comments.
But Mr Corbyn has now personally accused Mr Watson of talking "nonsense" as the Labour leadership contest grew increasingly intense.
In an interview with the Observer, the Labour leader said of Mr Watson: "I read about his letter to me in the media. And it appeared to be a rehash of a book (journalist) Michael Crick wrote 20 years ago about alleged entryism into the Labour Party at that stage.
"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."
Mr Corbyn also backed new members who used to support other parties.
"I want people to join for good motives," he said. "But if they have changed their political views or developed their political views, then surely that is a good thing."
Mr Watson hit back, insisting the evidence was "incontrovertible".
He told the newspaper: "The overwhelming majority of new members joined the Labour Party because they want to build a fairer and more equal society. But there is clear and incontrovertible evidence that a small group of Trotskyite activists have taken leading roles in the Labour Party or are seeking to do so.
"They are also explicitly targeting Young Labour and Labour student clubs with the aim of recruiting new members. That is beyond dispute. We can't deal with this problem until we acknowledge it exists."
Mr Corbyn also offered his first public comments on the Court of Appeal's judgment to bar around 130,000 members from voting in the leadership contest in a ruling which upheld the initial decision of Labour's ruling body.
Many of those affected are believed to back Mr Corbyn rather than his rival Owen Smith and the leader's campaign team reacted with fury at the court's decision to exclude those who had not joined the party by January 12 and held membership continuously until July 12.
When asked, Mr Corbyn would not say he had full confidence in Iain McNicol, Labour's general secretary, who led the appeal which was brought by the party's National Executive Committee (NEC).
The Labour leader said: "People joined the Labour Party in order to take part in the party and were specifically told that they were able to vote in the leadership election and it was decided by the High Court that they could.
"The Appeal Court has said they can't and I would imagine that those who brought the case will be considering whether or not to take it to the Supreme Court. I have no idea what their decision will be ...
"We will receive a report from Iain about the process that has gone on over the last few months. And the NEC will no doubt ask him questions and he will probably give answers on it. But let's look at that when the new NEC takes over."
Asked twice if he had full confidence in Mr McNicol, Mr Corbyn repeatedly stated: "I have been happy to work with Iain McNicol since I became leader."
Meanwhile, Mr Corbyn's team was compared to Nazi brownshirt stormtroopers by Jewish Labour donor Michael Foster, who failed in his attempt to overturn a separate NEC decision to automatically grant Mr Corbyn a place on the ballot paper for the leadership.
Writing in the Mail on Sunday about the Court of Appeal's Friday ruling and his own unsuccessful foray into the courts, he said: "The courts decided that the rules as they stand allowed it (Mr Corbyn to be automatically included on the leadership ballot).
"This decision advantaged Corbyn and his Sturm Abteilung (stormtroopers), but on Friday afternoon the Appeal Court handed down a big decision for British democracy (to exclude 130,000 members).
"It disallowed the attempt by arriviste followers of Corbyn to flood the Labour electoral college. This caused the mask of reasonableness of the Corbynista leadership to slip even further."