An investigation by the equality watchdog into antisemitism in the Labour party is the most shameful moment in the party’s history, a leading frontbench Labour MP has said. The Equality and Human Rights Commission’s report is due to be published on Thursday after an 18-month investigation into claims of anti-Jewish racism in the party during Jeremy Corbyn’s leadership. Asked if the investigation was the most shameful moment in the party’s history, Jonathan Ashworth, the shadow health secretary, agreed that “it probably was, yes”. Speaking on Times Radio on Wednesday morning, Ashworth said: “A lot of this was about the fact that there was just a refusal to acknowledge the issue. “I obviously don’t know what’s in the report, because it’s confidential, but that was a shameful period in our history, and we have to be clear that we are never going back to that, and we will do everything we can to repair relations with the Jewish community who are understandably and quite rightly hurt by the Labour party’s failure to deal with this in recent years.” The EHRC launched the review after whistleblowers alleged that the party was institutionally antisemitic in its handling of complaints and within local party structures. The referral came from a number of Jewish groups including the Campaign Against Antisemitism and the Jewish Labour Movement. Key allies of Corbyn are expected to launch a staunch defence of their record when the report is published. Karie Murphy, Corbyn’s former chief of staff, whom several former party staffers have accused of meddling in antisemitism cases, has said the handling of antisemitism disciplinary cases improved during Corbyn’s tenure. “Under Jeremy Corbyn’s leadership, antisemites were removed from the Labour party more quickly, transparently and effectively than ever before. As his former chief of staff, I’m proud of that record,” she wrote in a piece for the Guardian on Monday. Gideon Falter, the chief executive of Campaign Against Antisemitism, said there must be consequences for Labour staffers if the report is highly critical. “The party must be forever changed after this episode so this can never happen again,” he said. “Those responsible remain in the party and must be held to account if Sir Keir Starmer is to tear antisemitism ‘out by its roots’, as he has promised. The EHRC’s report is a pivotal moment in this corrective process, which is why we called in the EHRC and were the originating complainant in its investigation. “The EHRC has considered a great deal of evidence from us and we will have more to say when the report is published.”