Momentum founder Jon Lansman has admitted that parliament’s new Independent Group of MPs is a threat to Labour, as his organisation’s Corbyn-backing activists mobilise in a bid to force by-elections in defectors’ seats.

In an exclusive interview with The Independent, Mr Lansman said that while he believes the new group is guaranteed to fail under its own shortcomings, Momentum will seek to minimise the political cost to Mr Corbyn by accelerating its downfall.

It means the 40,000-strong group will target its campaign fire on unseating the eight Labour MPs who have set up the group – including the likes of Chuka Umunna, Mike Gapes and Joan Ryan.

His comments follow the most dramatic schism experienced by the Labour Party since the formation of the Social Democratic Party (SDP) in the 1980s, which has seen nine MPs quit the party in five days while delivering withering criticisms of Mr Corbyn’s leadership, and his handling of antisemitism and Brexit.

In a rally in Nottinghamshire on Saturday, the leader accused the departing MPs of “not understanding” where the Labour Party had come from, as he said they had made a “bad mistake”.

Which MPs defected to form Change UK? Show all 12 1 /12 Which MPs defected to form Change UK? Which MPs defected to form Change UK? Change UK Details on the individual MPs are in the following photos Reuters Which MPs defected to form Change UK? Heidi Allen Anti-Brexit MP for South Cambridgeshire resigned from the Conservative party on February 20 PA Which MPs defected to form Change UK? Chuka Umunna MP for Streatham since 2010 and prominent People's Vote supporter PA Which MPs defected to form Change UK? Anna Soubry The prominent anti-Brexit MP for Broxtowe resigned from the Conservative party on February 20 PA Which MPs defected to form Change UK? Luciana Berger MP for Liverpool Wavertree since 2010, resigned from the Labour Party over bullying and anti-semitism PA Which MPs defected to form Change UK? Sarah Wollaston Anti-Brexit MP for Totnes resigned from the Conservative party on February 20 PA Which MPs defected to form Change UK? Joan Ryan MP for Enfield North resigned from the Labour party on February 19 citing its tolerance of a "culture of anti-Jewish racism" PA Which MPs defected to form Change UK? Ann Coffey MP for Stockport since 1992 Chris McAndrew / UK Parliament Which MPs defected to form Change UK? Gavin Shuker MP for Luton South since 2010 Getty Which MPs defected to form Change UK? Chris Leslie MP for Nottingham East since 2010 PA Which MPs defected to form Change UK? Mike Gapes MP for Ilford South since 1992 PA Which MPs defected to form Change UK? Angela Smith MP for Penistone and Stocksbridge since 2010

At the close of yet another week of turmoil in British politics, Mr Lansman said he “didn’t want anyone” to leave after Ian Austin became the ninth MP to resign from Labour on Friday morning, adding that it was “tragic” and personally “upsetting” to see the Jewish MP Luciana Berger leave the party over antisemitism. Mr Austin chose not to join the Independent Group.

Mr Lansman also sought to deflate the hype around the new grouping of former Labour and Tory MPs, claiming that they are “far less likely to break the mould of British politics than the SDP”.

“The lack of a credible leader is a serious matter but actually the SDP had three credible leaders out of four when it launched,” he added.

“The SDP had people who were political heavyweights. These are people who are political lightweights. They had a very clear programme. These people do not.”

While he said “we should not exaggerate the threat”, he then went on: “Obviously the threat as it is, is that whilst it won’t succeed itself which is absolutely guaranteed in my view, it may damage us.”

Obviously the threat as it is, is that whilst it won’t succeed itself which is absolutely guaranteed in my view, it may damage us Momentum chief Jon Lansman

Mr Lansman said Momentum – created in the wake of Mr Corbyn’s first leadership victory to support his left-wing policy agenda – will now use its campaigning force in the constituencies of the MPs who left the party this week to build pressure for by-elections.

The organisation will hold canvassing events in Streatham, Stockport and Penistone, and Stocksbridge – the constituencies of former Labour MPs, Chuka Umunna , Ann Coffey, and Angela Smith.

All have rejected calls from Labour’s top team to test the support of their local electorates.

“We’re going to be collecting signatures for petitions in those constituencies,” he said. “Calling for by-elections. Having been elected on the basis of the manifesto that they are now rejecting – do the honourable thing.”

During a surprise press conference on Monday, Ms Berger was one of seven MPs to announce they were quitting Labour to form the Independent Group in the Commons.

In her brief remarks, the MP for Liverpool Wavertree had come to the “sickening conclusion [Labour] is institutionally antisemitic”.

Asked whether he believed more resignations were to follow in the coming days, Mr Lansman replied: “I don’t know the individuals concerned well enough. I hear rumours and they may have some foundation – I hope not.”

Shortly after the Independent Group was formed, polling released by YouGov suggested that if it existed as a political party and put candidates up at the general election it would enjoy 14 per cent of votes from the British public.

“Compared to our current voting intention figures, it’s clear that the Independent Group are taking support from the Conservatives, Labour and Liberal Democrats – though it’s Labour who take the biggest knock,” the pollsters said.

Jeremy Corbyn on the Independent Group: 'They were elected on the same manifesto as me'

But Mr Lansman dismissed the findings, adding: “I think polls taken on the basis of today’s headlines for an organisation that has not said anything about what it stands for have to be taken with a great deal of scepticism. I think it will be some time before the polls yield anything like accurate information.”

Speaking on Friday, the Momentum founder also said he was “surprised” by the decision of the Labour Party to allow Derek Hatton back into ranks 34 years after he was expelled – only to suspend him two days later after a tweet emerged from 2012, which said Jews with “humanity” should criticise Israel.

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Kicked out of the party in the 1980s by then leader Neil Kinnock, the former deputy leader of Liverpool City Council caused controversy at the time for overseeing an illegal “deficit” budget in protest at Margaret Thatcher’s Conservative government.