The Jewish Labour Movement passed a motion of no confidence in Jeremy Corbyn’s leadership on Sunday as the party came under renewed pressure over its handling of antisemitism allegations.

Labour disputed a report in the Sunday Times, which drew on leaked internal documents, that its system for dealing with complaints has been beset by delays, inaction and interference from the leader’s office.

But both the party and its leader were dealt a blow when the AGM of the Jewish Labour Movement – which has 2,000 members and has been affiliated for almost a century – voted to describe the party as “institutionally antisemitic”.

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A motion expressing no confidence in Corbyn and describing him as being unfit to lead the country was also passed unanimously after speeches by figures including the Labour MPs Ruth Smeeth, Louise Ellman and Margaret Hodge, who have clashed with the leader on the issue of antisemitism.

It was passed despite a plea hours earlier by the shadow attorney general, Shami Chakrabarti, for the Jewish Labour Movement “not to personalise” the issue.

“My plea to the Jewish Labour Movement is to stay in the Labour movement and to tackle racism together, not to personalise it and make it about Jeremy Corbyn, because he is one person and he won’t be leader forever,” Chakrabarti told Sky News’s Sophy Ridge on Sunday.

Tom Watson said: “I hear the message the Jewish Labour Movement and the Jewish community are sending our party. We have utterly failed to deal with antisemitism in our ranks. It’s unacceptable and shameful.

“I reiterate my personal commitment as deputy leader to use all my powers to rid our party of this scourge.”

Emily Thornberry, the shadow foreign secretary, told Politico on Sunday evening: “I’m completely disgusted this is continuing to happen. Some of the things that were said just turn my stomach, and the idea that these people are still in my Labour party disgusts me.”

The development came as sources close to the leader and senior sources elsewhere in the party were at loggerheads over the accuracy of figures cited by the Sunday Times, which reported that Corbyn’s office had been involved in approving, delaying or blocking at least 101 complaints of antisemitism.

Out of 863 complaints, 454 were said to be unresolved, including 249 where the party had not started an investigation. Out of 409 cases where a decision was reached, 191 members faced no further action, 145 received a formal warning and fewer than 30 were expelled.

Sources close to the leader’s office said that the figures had existed on a spreadsheet document but that they were inaccurate. The 101 figure was said to be a “massive overstatement”.

However, senior sources elsewhere within Labour said that they could vouch for the accuracy of the figures, which appear set to be the focus of renewed clashes this week on the part of different factions at the top of the party.

Labour claimed the figures, which were reportedly drawn from a hard drive of emails and a confidential database last updated on 8 March, were not accurate and that lines had been selectively leaked from emails to misrepresent their overall contents.

A spokesperson said: “The information which was leaked to the Sunday Times does not reflect the full details of cases and action taken and is not up to date. It is not taken from our complaints tracking system and it contains a number of inaccuracies. It vastly overstates the number of complaints allegedly ‘sent to LOTO’. More cases have received decisions on suspension than reported, so the number of ‘outstanding’ cases is also wrong.

“The Labour party recently released figures and we are committed to thorough and consistent handling of complaints. We report cases regularly to the NEC. Reporting misleading and inaccurate information misleads the public and undermines effective action against antisemitism.”

Watson described the reports in the Sunday Times as “shocking and depressing reading”. Since raising the alarm several weeks ago over what he believed was a lack of transparency over antisemitism, he has been sending Formby an updated list of complaints on a regular basis but was said to be growing impatient with a lack of response from her.

Members investigated for posting online comments such as “Heil Hitler” and “Jews are the problem” had not been expelled, even though the party received the complaints a year ago, according to the Sunday Times.

It reported that Labour had failed to expel Sarah Reeves, a former candidate whom party officials have accused of being “supportive of Hitler and Mein Kampf” in her Facebook postings. Reeves, whose Facebook feed also contains explicit support for the far-right activist Tommy Robinson and conspiracy theories such as the claim that 9/11 was an “inside job”, told the Guardian on Sunday that she had yet to receive notification from Labour about when a hearing into her cases would be heard.

Pam Bromley, who was reported to have been let back into the Labour party after claiming last year that Theresa May had plotted the Manchester bombing and complained of “Jewish” media attacks, referred queries from the Guardian to the Labour press office.

The documents were reported to show that a trade union official was readmitted to the party after being accused of sharing material saying “Jewish Israelis” were behind 9/11. In another case, a Labour official ruled that a council candidate accused of describing Jewish MPs as “Zionist infiltrators” met the threshold for suspension, but then ruled he should not be suspended as he was a candidate, the paper said.

Jonathan Goldstein, the chair of the Jewish Leadership Council, a charity that brings together British Jewish organisations, said it was simply impossible for Labour members to ignore the degree to which racism had infected the party.

“These emails lay bare the extent of the corruption and those who have covered it up must be held to account without delay and relieved of their duties,” he said.