Labour’s chief whip has told Jeremy Corbyn to “unequivocally, unambiguously and without reservation” apologise for his claims that the extent of antisemitism in the party had been “dramatically overstated”, which led to his membership being suspended. The party’s governing body readmitted Corbyn last week but Labour’s leader, Keir Starmer, has refused to restore the whip to his predecessor, effectively suspending him from the parliamentary party. The chief whip, Nick Brown, has said the suspension will last at least three months while an investigation takes place into whether Corbyn has broken the MPs’ code of conduct. In a new letter released by Brown on Monday, the chief whip told Corbyn he must apologise for his comments made in the wake of a report on antisemitism in the party under his leadership by the equalities watchdog. Brown said the comments made in response to the Equality and Human Rights Commission (EHRC) report caused “distress and pain” to the Jewish community and should be deleted from the former Labour leader’s Facebook page. Brown said Corbyn must also commit to supporting the party’s efforts to comply with recommendations by the equalities watchdog. He said the suspension was related to parliamentary rules “that Labour MPs conduct themselves at all times in a manner consistent with membership of the Parliamentary Labour Party, including doing nothing that brings the Labour party into disrepute”. Multiple sources have previously told the Guardian that conversations took place in recent weeks between representatives of Corbyn and key figures in Starmer’s office, including his chief of staff, Morgan McSweeney, ahead of Corbyn being readmitted to the party. Corbyn’s side believed the outcome of those negotiations was that the party’s governing national executive committee (NEC) would issue Corbyn with a lesser punishment, but sources close to Starmer have underlined that Corbyn had been repeatedly urged to retract last month’s comments – which he had not done despite issuing clarifications. In Monday’s letter, Brown said Corbyn’s statement “in particular, the implications that this form of racism is ‘exaggerated’ and that it is media reporting of that racism, rather than the actual impact on its victims, that hurt Jewish people – caused significant distress to many, in particular Jewish members of the Labour party and the wider Jewish community”. Brown said he would be happy to meet Corbyn soon to discuss the potential restoration of the whip. “The EHRC’s conclusion that the Labour party had broken the law by committing unlawful acts is a shameful indictment for our party,” Brown wrote. “There is a legal requirement to correct where the Labour party has acted unlawfully and a moral obligation to implement all the recommendations of the EHRC report. Keir Starmer has been unequivocal that antisemitism has been a stain on our party and that we must tear out this poison by its roots.” Members of the NEC will meet on Tuesday in what is expected to be a showdown following Corbyn’s reinstatement and the refusal of Starmer to restore the whip. Last week, in the most open declaration of civil war in the party yet, 14 left-aligned members of the NEC said the decision to withhold the whip from Corbyn was a “deliberate political interference in the handling of a complaint” and called on the general secretary, David Evans, to intervene.