Labour leader Jeremy Corbyn has been criticised for attending an event organised by Jewdas – a left-wing Jewish group critical of more mainstream Jewish organisations.
It comes hours after Mr Corbyn promised “watertight” investigations into claims of anti-Semitism within his party.
One Labour MP called his actions “irresponsible and dangerous”.
On Monday, Momentum, the movement backing Mr Corbyn, said the party had “failed” on anti-Semitism.
Jewdas has accused the Jewish Board of Deputies, Jewish Leadership Council and Jewish Labour Movement of “playing a dangerous game” and previously dismissed anti-Semitism allegations as right-wing smears.
A spokesman for the Labour leader confirmed Mr Corbyn had attended the event, marking the Jewish festival of Seder, in a personal capacity and not on behalf of the party.
BBC assistant political editor Norman Smith said there was “nothing remarkable” about Mr Corbyn attending the event in his constituency alongside a group he shares many views with – such as its anti-capitalist stance and pro-Palestinian views.
However, he said it had raised questions about why the Labour leader did not go to a Seder celebration by a more mainstream organisation, with some of his critics seeing his attendance as “almost playing down the significance of the current row over anti-Semitism”.
‘Baiting the mainstream’
In a tweet, Labour MP John Woodcock said meeting the group gave the message that “extreme views are OK”.
Labour’s Angela Smith said Mr Corbyn’s attendance was a “blatant dismissal” of the case to tackle anti-Semitism in the party.
Meanwhile, the chairman of the Campaign Against Anti-Semitism, Gideon Falter, said Mr Corbyn’s claims to remedy anti-Semitism within the party and his appearance at this meeting made him a “duplicitous” man.
The chief executive of the Holocaust Educational Trust, Karen Pollock, said on Twitter it was “mocking and disrespectful”.
The party has been engulfed in a row about attitudes towards the Jewish community after it emerged there was a backlog of some 70 complaints of anti-Semitism within the party that had not been dealt with.
Mr Corbyn has previously apologised for the “hurt” in the failures to address complaints and insisted the party does not tolerate anti-Semitism in any form.
Labour has dropped one council candidate over allegedly sharing posts on social media urging followers to “question the Holocaust”.
In its statement on Monday, Momentum’s national co-ordinating group acknowledged the failure to deal with cases in a “sufficiently decisive, swift and transparent manner”.
While it suggested that Mr Corbyn’s opponents might be “opportunistically using this issue as a way to undermine his leadership”, that did not diminish “our responsibility to challenge anti-Semitism whenever and wherever it occurs”.
Later, the party’s new general secretary will start her role to ensure allegations of anti-Semitism are dealt with effectively.
In an interview with Channel 4 News, Mr Corbyn said the issue would be a top priority for Jennie Formby and insisted “absolutely strong and watertight” investigations would take place.