Labour has a much larger number of members who hold “hardcore antisemitic opinions” than previously thought, Momentum’s founder has said.

Jon Lansman, who is chair of Momentum and also a member of Labour’s national executive committee (NEC), said antisemitism was a widespread problem in the party and that he felt “regret, sadness and some shame” at the decision of the Jewish MP Luciana Berger to quit the party last week.

He told BBC Radio 4’s Today programme that the party was not institutionally antisemitic, and instead attributed the problem to “conspiracy theorists” who had joined the party since 2015.

“The party trebled in size,” said Lansman, who is Jewish. “Amongst those are members attracted towards conspiracy theories.” He added: “The Tory party is a smaller party and an elderly party and the role of social media in fomenting and spreading some of the poison is therefore more of a problem in the Labour party.”

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Facebook groups in particular can become arenas in which a minority with hardcore views can become amplified, Lansman said. He also defended the Labour leader, saying “Jeremy Corbyn has been an anti-racist all his life and I absolutely believe his commitment to that.”

Lansman’s intervention came after the party’s deputy leader, Tom Watson, said on Sunday there was “a crisis for the soul of the Labour party” and disclosed he had forwarded 50 complaints about antisemitism to Corbyn.

Lansman said the NEC was looking at more cases of antisemitism. When pressed as to whether Corbyn could provide stronger leadership to combat antisemitism, as Watson had urged, he said: “I’m not sure about him taking personal responsibility for cases. I think if he did that, some people might argue that that would be inappropriate.”

He added: “I don’t want any Jewish member in the party to be leaving. We are absolutely committed to making Labour a safe space.”

On Twitter, Lansman later added: “Just last Friday we referred 19 out of 35 cases reviews to the NCC almost all with a strong recommendation for expulsion. Of Labour’s 500,000 members perhaps a few hundred are hardcore antisemites. If we improve our processes, we can make sure they are kicked out of the party.”

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Lansman, asked about the wider problems that Labour was facing, said he believed the party would hold together in the face of resignations over issues including antisemitism, Corbyn and Brexit. He said a proposal by Watson that a new internal grouping for MPs from the social democrat tradition who are not represented in the shadow cabinet should be considered.

“I welcome members with a common view,” Lansman said. “We’re a pluralist party.”

On Sunday, a Labour spokesperson said the party “takes all complaints of antisemitism seriously” and that “all complaints about antisemitism are investigated in line with our rules and procedures”.