Jeremy Corbyn accuses Tom Watson of making 'nonsense' Trotsky claims
Jeremy Corbyn has dismissed claims by his deputy that hard-left activists are trying to infiltrate the Labour Party ahead of the leadership vote.
Mr Corbyn said Tom Watson's suggestion that "Trotsky entryists" are manipulating young party members to boost his support were "nonsense".
But Mr Watson responded saying there was "clear and incontrovertible evidence" to back his claims.
The Labour leader is embroiled in a contest with challenger Owen Smith.
In an interview with the Observer, Mr Corbyn 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 - I think he knows it's nonsense."
Mr Watson said he believed the "overwhelming majority" of new members joined the Labour Party because they want to build a "fairer and more equal society".
But he said: "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."
BBC political correspondent Carole Walker said "the open row" between the Labour leader and his deputy "is a further sign of deep divisions within the party".
There has been Labour in-fighting over the massive influx of new members that have signed up to the party since Mr Corbyn became leader last September.
Mr Watson told the Guardian earlier this week that Labour was being infiltrated by "Trotsky entryists" who had "come back" to bolster Mr Corbyn.
The Labour leader's campaign team hit back accusing Mr Watson of "peddling baseless conspiracy theories" but the deputy leader then wrote a letter to Mr Corbyn urging him to take action against infiltrators.
What is a Trotskyist?
Trotskyism has its origins in early 20th Century Russian politics and the path pursued by one of the founders of the Soviet Union, Leon Trotsky.
Trotsky was the head of the Red Army and a key player in the violent revolution that toppled the Russian tsar and established the world's first socialist state.
But he split with the other revolutionary leaders Lenin and Stalin, who believed they could create a socialist society in their own country without a world revolution.
Trotsky believed his country could achieve socialism only if the working classes around the world rose up as one to overthrow the ruling classes - the doctrine of "international socialism".
In his Observer interview Mr Corbyn also refused to express full confidence in the party's general secretary Ian McNichol who was a key figure in recent legal action which stopped 130,000 of the new members from voting in the leadership contest.
Mr Corbyn said Mr McNichol would face questions over the events of the last few months.
"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 that was decided by the high court that they could," he said.
"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 think that people should have the right to take part and that is surely what democracy is about."