Jon Lansman, founder of the influential pro-Jeremy Corbyn group Momentum, called today for a “widespread programme of training and education” within the Labour Party on antisemitism.

“We have all realised the extent of the problem,” he told Radio 4’s Today. "We're all, I think, tired of too many people arguing it is all smears. It isn’t”.

But Mr Lansman, who is Jewish and a member of Labour’s national executive committee, defended Mr Corbyn’s attendance at a  Seder last night held by the radical Jewish group Jewdas.

He said the Labour leader was attending an event with members of his Islington constituency. “It was his night off. His office didn’t know he was there.”

Jewdas is a far-left group which called last week’s protest against antisemitism in Labour to be “faux outrage” and accused the groups who organised it of “playing a dangerous game with people’s lives”.

While Mr Corbyn’s attendance has been criticised by some Labour MPs, Mr Lansman said the Labour leader has been seeking to meet mainstream Jewish groups.

But Jonathan Goldstein, chairman of the Jewish Leadership Council, took issue with Mr Corbyn’s actions.

While Mr Corbyn had told the JLC and the Board of Deputies that he was an “ally” in the fight against antisemitism, “he in his first act towards the Jewish community has gone to sit with a group who describe the Jewish Leadership Council and Board of Deputies’ actions as being a cynical ploy”.

The Labour leader instead, he told the Today programme, should have “stuck up” for those of his MPs who were being vilified for attending Monday’s demonstration against antisemitism in Westminster.

Mr Goldstein explained that the Board’s and JLC’s response to a request from Mr Corbyn for an urgent meeting had not contained pre-conditions.

But the Labour leader needed to make clear that he considered allegations of antisemitism within Labour were not smears.

Since the JLC and Board had responded, there had been “silence”, he said.

This is deliberately baiting the mainstream Jewish community days after they pleaded with him to tackle antisemitism. And he must know that meeting them now will give his members the message that the group’s extreme views are ok. Irresponsible and dangerous https://t.co/NORQQiFq8J

— John Woodcock (@JWoodcockMP) April 2, 2018

Mr Corbyn’s appearance at the Jewdas Seder was revealed by the political website Guido Fawkes.

In a statement attacking the rally organised by the Board and JLC last week, Jewdas said it was “the work of cynical manipulations by people whose express loyalty is to the Conservative Party and the right wing of the Labour Party.

“It is a malicious ploy to remove the Leader of the Opposition and put a stop to the possibility of a socialist government.”

The group ended its statement by saying “f*** you all. Chag Pesach sameach”.

But Labour MP Wes Streeting criticised Mr Corbyn for going to the Seder, saying: “This demonstrates either extraordinarily bad judgement or a deliberate affront to the majority of British Jews. Probably both.

“It calls into serious question the sincerity of every public statement Jeremy Corbyn has made on antisemitism during the past week, just as many of us hoped we had begun to make progress.”

Labour MP John Woodcock said going to the Seder was “deliberately baiting the mainstream Jewish community days after they pleaded with him to tackle antisemitism”.

Karen Pollock, chief executive of the Holocaust Educational Trust, called it “mocking” and “disrespectful”.

Mr Lansman said antisemitism education was needed for Labour members to understand the issues because of an “unconscious bias”.

He had become more aware, he said, from his own Facebook feed that there were people who were dismissing antisemitism in which Islamophobia would not have been dismissed.

“That reveals unconscious bias,” he said.

There was “clearly real concern” about antisemitism in the Jewish community - “we have to take it seriously,” he said.

Every allegation of antisemitism had to be taken seriously, he said, even when its motivation may have been opportunistic in an attempt to undermine Mr Corbyn’s leadership.

Ivor Caplin, a spokesperson for the Jewish Labour Movement (JLM), said:

“Jeremy Corbyn’s decision to spend his evening with an organisation that said ‘f*** you all’ to Jews who have serious and well-founded concerns over antisemitism in the Labour Party, has truly topped off the worst week on record of awful relations between the Labour party and the Jewish community.

“When we called on the party to show moral leadership, and take decisive action to stamp out antisemitism, this is not what we had in mind.

“Jewish party activists have spent the weekend knocking on doors for local candidates for council elections across the country. They do not deserve the indignity of our leader making this situation even worse.”