A trade union leader has been recorded suggesting that Israel "created" the anti-Semitism row in the Labour Party.
In a recording published by the Independent, PCS general secretary Mark Serwotka suggested the country had created the story to hide what he called its own "atrocities".
Labour Against Anti-Semitism said Mr Serwotka had brought the entire TUC into disrepute and he should resign.
The PCS said he had made the point that anti-Semitism must be opposed.
Marie van der Zyl, president of the Board of Deputies of British Jews, said: "By trading in conspiracy theories and stock anti-Semitic tropes, Mark Serwotka has demeaned his office.
"The only reason Labour's anti-Semitism crisis remains in the media is Labour's failure to deal with it.
"Mr Serwotka should apologise or face disciplinary action from the TUC and the Public and Commercial Services Union."
And Matt Zarb-Cousin, Jeremy Corbyn's former spokesman, said the comments were wrong and "very unhelpful".
The Independent reported that Mr Serwokta had told an event organised by the Palestinian Solidarity Campaign at the TUC this week that there was no place for anti-Semitism in the Labour movement.
"I think it's unfortunate that the Labour Party allowed a lot of this to drag on, in a way that actually didn't help anybody," he said.
But he cited the controversial decision by US President Donald Trump to move the US embassy from Tel Aviv to Jerusalem and the unspecified killing by Israeli troops of "dozens of Palestinians… unarmed, innocent civilians" as among the real issues being distracted from by "a summer of asking ourselves whether leading Labour movement people are in any way anti-Semitic?".
"Now I'm not a conspiracy theorist, but I'll tell you what - one of the best forms of trying to hide from the atrocities that you are committing is to go on the offensive and to actually create a story that does not exist for people on this platform, the trade union movement or, I have to say, for the leader of the Labour Party," he said.
Mr Serwotka is general secretary of the largest civil service trade union, the Public and Commercial Services Union, a post he has held since December 2000. He was re-elected in 2005, 2009 and 2014.
In 2015 he was among people banned from voting in Labour's leadership election on the grounds they did not share the party's "aims and values" - new rules had allowed non-members to vote as "registered supporters".
In 2016, he rejoined the Labour Party after 25 years, saying Jeremy Corbyn's leadership offered a "genuine break from the past".
On Wednesday it was announced that had been elected TUC president for the coming year.
Labour has been dealing with a row about the extent of anti-Semitism within the party for more than two years. A 2016 inquiry, carried out by Shami Chakrabarti, concluded that while the Labour Party was not overrun by anti-Semitism, there was an "occasionally toxic atmosphere".
But the row over the way the leadership has handled anti-Semitism allegations continued and dominated media coverage of the party over the summer.
There was much controversy over a new code of conduct the party adopted on anti-Semitism, amid criticism it did not go as far as the International Holocaust Remembrance Alliance's guidelines.
Veteran MP Frank Field quit the party's parliamentary group in August, saying the leadership was becoming a "force for anti-Semitism in British politics".
Earlier this month the party's ruling body agreed to adopt in full an international definition of anti-Semitism.
Mr Corbyn has apologised for hurt caused by anti-Semitism in the party and pledged to stamp it out.
He has stressed that people who hold anti-Semitic views "have no place in the Labour Party" and said people who use "anti-Semitic poison" are not his supporters, nor do they speak for him or the party.
A spokesperson for the Public and Commercial Services Union said: "Mark spoke at a Palestine Solidarity Campaign fringe event at the TUC - an organisation PCS is affiliated to.
"He made the point at the start of the meeting, that we need to oppose anti-Semitism in society and within the Labour movement.
"But we should not allow the issue of anti-Semitism to be used by people who are attempting to silence Palestinian voices as they legitimately struggle for their rights and a sovereign state."