JEREMY Corbyn has accused his deputy Tom Watson of talking “nonsense” and deliberately deceiving Labour members with his claims of “Trotsky entryists” manipulating young party members to boost support for the leader.

Corbyn delivered a stinging rebuke to Watson as Labour’s increasingly bitter leadership contest spilled over into a public spat between its leader and deputy leader after what had been a relatively peaceful public relationship between the two men.

The Labour leader, in one of his most strident responses to critics yet, dismissed claims of far-left infiltration of the party by Watson, adding “I think he knows it’s nonsense”.

However, Watson responded by saying there was “clear and incontrovertible evidence” to back his claims, which he made about Trotskyist groups such as the Socialist Party, formerly known as the Militant Tendency, seeking to influence the leadership contest.

Last week Watson had said that Labour was being infiltrated by “Trotsky entryists” who had “come back” to the party after decades outside it to bolster Corbyn.

Corbyn, who is embroiled in a leadership contest with Owen Smith after concerted efforts by Labour MPs to oust him, launched one of his most robust responses yet to internal critics, as well as his first real public criticism of Watson.

Corbyn said of 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.”

Corbyn also backed new members who used to support other parties such as the Greens and Liberal Democrats. He said: “I want people to join for good motives. But if they have changed their political views or developed their political views, then surely that is a good thing.”

But Watson said far left and Trotskyist groups were seeking to take over key positions within the party and were targeting young members in particular, a tactic associated with Militant which was banned from Labour in the 1980s.

Watson said: “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.”

Meanwhile, it emerged that Labour’s leadership election will go ahead with the exclusion of about 130,000 new members after five of them dropped a legal challenge against the decision to bar them.

On Friday the Court of Appeal ruled Labour was within its rights to stop some new members voting in the contest between Corbyn and Smith.

It overturned an earlier High Court ruling that the ban was unlawful.

The five members said they will not take their case to the Supreme Court.

The five, Christine Evangelou, Rev Edward Leir, Hannah Fordham, Chris Granger and “FM”, a teenage member, have raised £93,572 in donations but said they could not afford to take the case further.

In a message posted on their crowdfunding website, Fordham said: “But the case wasn’t in vain, although we didn’t succeed in reclaiming votes for the 130,000 disenfranchised members, we did win in the High Court, exposing facts which have spurred important conversations about the role of the Labour Party membership and the NEC.”

Meanwhile, leadership challenger Smith will today accuse the Tories of having a “secret plan to privatise the NHS” while stressing public spending on private healthcare providers has doubled since they took power.