Seasoned addicts of this column will, I hope, agree that I've never used it to advocate voting for a particular political party or a political person. I myself have never belonged to any political party, and I regard myself as a detached academic observer, grown ever more cynical as one birthday has followed another. But I have, on occasion, used this column to warn against supporting a particular party or person. I intend to do so again today.

Next May, Londoners will go to the polls to elect a mayor in succession to Boris Johnson. All the major parties have now chosen their candidates, but for my present purposes the candidate who requires very careful consideration is Sadiq Khan, currently Labour MP for Tooting and a former minister under Gordon Brown.

Khan, born and bred in London, is the son of a London bus-driver (there's absolutely no harm in that; I'm the son of a London warehouse operative). In 2009, having been appointed Minister of State for Transport, he became the first Muslim ever to attend a meeting of the Cabinet. At 44 years of age, Khan is politically ambitious, as he has every right to be. Last month, he won a decisive victory in the contest to become Labour's London mayoral candidate. On the face of it, Khan's philosemitic credentials look impressive. Last May, he penned a newspaper article proclaiming his ambition "to be a Muslim mayor that stands up for Jewish rights. Because I know what it's like to be a member of a minority group in London… I know what it's like to be discriminated against. That's why I'll never stop fighting racism in all its forms, including antisemitism."

All well and good (you might say). But there was more to come. Interviewed by the JC after his mayoral nomination victory , Khan demonstrated that he was well aware of London Jewry's antipathy to his Labour predecessor as mayoral candidate, Ken Livingstone. Vowing to repair Labour's relationship with London Jewry, "believe me" (he told the JC), "I won't be another Ken Livingstone. What's important to me is that we have zero tolerance of antisemitism. I will ensure there are sufficient police resources and the resources of City Hall to address this issue." It's true that Khan nominated Jeremy Corbyn as Labour leader. Shrewdly, however, he actually voted for Andy Burnham. He's been assiduous in visiting Jewish locations and meeting and trying to make friends with Jews. And he has pledged, if elected mayor, not to discuss foreign policy issues and to work closely with Jewish charities.

But there is another side to this equation. In January 2009, Kahn was listed as a speaker at an event in Tooting (hosted by Wandsworth Stop The War Coalition, of which Jeremy Corbyn was a co-founder) that had as its purpose the denunciation of Israeli policy towards Gaza. The following month, according to a local Tooting newspaper, Khan was a speaker at another demonstration, a march and rally called in support of Palestinian suffering in Gaza.

Sadiq Khan's philosemitic credentials do look impressive

What Khan himself said I do not know. But a spokesman for Wandsworth STWC, interviewed by the same local newspaper about the same event, announced that the objects of the campaign were "to lift the economic blockade by Israel, open check points to allow aid, and revoke export arms licenses to Israel. We're also boycotting Israeli goods." I should add that, at the rally, speakers are reported to have included that well-known friend of the Jews the Reverend Stephen Sizer.

I will agree at once that one swallow does not a summer make. But we are talking about several swallows, aren't we? Of course it's entirely possible for Khan to announce that now that he is a mayoral candidate he has resolved to turn over a new leaf. My advice to him is to consider doing just that. As he does so he should reflect on the fact that at the 2012 mayoral contest one polling organisation quizzed a sample of London voters generally over a wide range of issues, including "the poor relationship between Ken Livingstone and the Jewish community." Of those respondents who declared themselves first-preference Boris Johnson supporters some 40 per cent specifically identified Livingstone's attitude to Jews as either a "very important" or a "quite important" factor in propelling them to vote for his Conservative opponent.

Worth thinking about, Mr Khan, isn't it?