More than 20 former presidents of the National Union of Students, including three former cabinet ministers, have sent an unprecedented private warning to the organisation’s trustees, urging them to address concerns from Jewish students. The letter, which has been leaked to the Guardian, was sent to the union’s trustees by 21 former presidents dating back to the 1960s, including the former cabinet ministers Jack Straw, Charles Clarke and Jim Murphy, the shadow health secretary, Wes Streeting, and Maeve Sherlock. “This is not just a matter of protecting NUS’s reputation, but honouring NUS’s proud anti-racist principles,” the letter said. Others include the Sky News presenter Trevor Phillips, the Times columnist David Aaronovitch and former Labour MPs Lorna Fitzsimons, Stephen Twigg and Phil Woolas, as well as Shakira Martin, the first black woman to lead the NUS. New concerns were raised by Jewish students after the union invited the rapper Lowkey to a centenary event, after he said the media had “weaponised the Jewish heritage of [Ukrainian president] Zelenskyy” in order to ignore alleged far-right activity in Ukraine. Students who raised objections said they were told they could stay away in a safe space during his performance, though the NUS has denied suggestions they were segregated. Lowkey pulled out of the event amid the controversy. Complaints were also made by the Union of Jewish Students (UJS) about comments by the union’s president-elect, Shaima Dallali, including a post from when she was a young teenager, which read: “Khaybar Khaybar O Jews… Muhammad’s army will return Gaza”, referencing a 628 massacre. She has since apologised for the post. Other social media posts are being investigated by the NUS board. The letter was revealed as the NUS board called a crisis meeting for Wednesday over the row. MP Robert Halfon, the Conservative chair of the education select committee, said he had referred the NUS to the Charity Commission over the issue. The higher education minister Michelle Donelan last week said she was also considering reporting the NUS to the commission, and that the government could suspend engagement with the union over the allegations. Donelan also called on student unions across the country to “consider disaffiliation, unless matters improve quickly”. In response to Halfon and Donelan’s criticism, the union said: “NUS is taking antisemitism allegations seriously. There is no place for antisemitism within the student movement. We have unreservedly apologised for the concern and worry caused in recent weeks, and are working to address any wrongdoing and rebuild trust. “The board are meeting to instigate our robust internal procedures including considering appointing an independent external party to support with this. If we find that action needs to be taken we won’t hesitate to take it, as we have previously. For information, NUS is not a charity and we aren’t funded by public money. However, we gladly set ourselves the highest bar in terms of our conduct. We welcome the opportunity to work with politicians, as we do with various government officials and advisors.” The leaked letter to the current NUS president, Larissa Kennedy, as well as the trustees, said there was an urgent need to take the complaints more seriously. “We are writing to you privately as former presidents with serious concerns about antisemitism, the safety and treatment of Jewish students at NUS events and within your democracy, and the way in which NUS is responding to these concerns,” the letter said. The letter said it was “clear NUS has a serious and significant problem”. The former presidents said NUS should issue “a full and unreserved apology” resolving to rebuild relationships with UJS and Jewish students and launching an independent investigation into antisemitism within the organisation, including statements and tweets made by current and incoming officers. “We hope that the trustees understand the gravity of the crisis facing the organisation, the reputational damage it is suffering, and the potentially existential threat that de-recognition of NUS poses to the future of the organisation – and your responsibility to act,” the letter said.