Sadiq Khan ought to be the most popular Labour politician in the UK. He has the largest personal mandate of any elected politician in British history. His victory in May wasn’t just a great triumph for Labour, it was a triumph for hope and decency over one of the nastiest dog-whistle campaigns that London has ever seen. His first hundred days have demonstrated the virtue of winning elections, putting Labour values into practice by cutting the cost of transport, putting additional capacity in to community policing and launching the most ambitious plan to tackle poor air quality that London has ever seen.

Instead, London’s new Mayor was booed by an audience of 4,000 people in Kilburn this weekend at a London rally supporting Jeremy Corbyn’s bid to be re-elected as Leader of the Labour Party. We should not be surprised by this. Since Corbyn’s election as Leader, the ‘kinder, gentler’ politics he promised has only been offered to those who offer their unwavering loyalty to the cult of the Leader.

What has been surprising has been the sinister antisemitic abuse that Sadiq Khan has received on social media. Like the Twitter user who posted a picture of Sadiq Khan breaking his fast at shul wearing a kippah, asking ‘who owns you Sadiq Khan?’ Or the Twitter user who stated that Khan ‘spends his time writing articles to help his masters in Tel Aviv’. Or the article headed ‘Sadiq Khan is a Zionist, the friends of Israel group are trying to ruin Corbyn’, which described him as a backstabber, accompanied by a photo of the Mayor speaking in front of the London Jewish Forum banner.

Not all of Khan’s poisonous detractors have used the term ‘Zionist’ as a label of abuse against the first ever Muslim mayor of a major western city. One Twitter user remarked: ‘Sadiq Khan is not, as such, a Zionist, but a ruthless person of Pakistani Muslim origins who has decided that he needs Jewish influence.’ So many bigoted tropes in under 140 characters.

Antisemitism in and around the Labour Party didn’t begin with Jeremy Corbyn’s leadership. The Labour Party, like any major political party, isn’t immune from the prejudices of the society it serves. Nor do I believe that the majority of Labour Party members, or even the majority of Corbyn supporters, are Jew-haters. But the toxic climate created by Corbyn’s leadership has left antisemites emboldened.

The Sunday Times report about the claims made by Josh Simons, a former policy advisor to Corbyn, about his experiences in the Leader’s office give an indication as to how deep the rot has set in at the top of the Labour Party. The Sunday Times reported that Simons submitted evidence to the Chakrabarti Inquiry into Labour’s antisemitism problem that included claims that a member of Corbyn’s office talked about a “Jewish conspiracy”, that the Leader’s office had prepared for a meeting with the Board of Deputies with “flippant disdain” and that he had been subjected to an “inquisition” about his own views on Israel by a senior member of Corbyn’s team. Worryingly, none of these claims about the culture of the Leader’s office made it into Shami Chakrabarti’s published report.

Sadiq Khan’s treatment demonstrates that antisemites on the left don’t confine their hateful abuse to Jewish politicians, but my Jewish colleagues have experienced the worst of it. The Party of Manny Shinwell, Ian Mikardo and Leo Abse, has become one in which Luciana Berger, Louise Ellman and Ruth Smeeth receive online abuse and death threats. Small wonder it has left lifelong servants and supporters of the Labour Party like Lord Parry Mitchell contemplating cutting up their membership cards if Corbyn wins again.

As ballot papers land and more than half a million Labour Party members and supporters prepare to choose our next Leader, there is an opportunity to turn the page on this dreadful chapter in Labour’s history. The re-election of Jeremy Corbyn would be a recipe for further division and hostility that neither the Labour Party nor the country can afford.

Sadiq Khan embodies the best of what Labour’s traditional values can achieve. The story of the council estate boy who went on to become a human rights lawyer, a government minister and the first Muslim mayor of London embodies the vision of the society that my Party wants to build: one where ability, not background, determines where people end up. He has called on Labour members to vote for Owen Smith. Let’s hope they heed his advice.