It is impossible to read Ian Hislop's account of his conversation with Julian Assange without feeling very uncomfortable. On the face of it, this is a classic example of antisemitism. And although Mr Assange denies it, he does not explain why Mr Hislop would have chosen to put his reputation on the line in such a public manner.

I was made particularly uneasy because I have worked with Julian Assange in the past and been a supporter of the work of WikiLeaks. In all my dealings with Julian, I never heard him express any antisemitic sentiments.

He did not even have the usual conspiracy theorist's tendency to place all the wrongs of the world at the door of Israel.

It is hard to imagine how much pressure he must be under. But it is simply unhinged to charge Index on Censorship chief executive John Kampfner, the Guardian's veteran investigative reporter David Leigh and its editor Alan Rusbridger with being part of a Jewish conspiracy. As it happens, none are even Jewish, although Messrs Kampfner and Leigh have Jewish fathers.

It is every bit as ridiculous as claiming, as some have done, that Julian Assange is an agent of Mossad because the WikiLeaks cables did not provide enough evidence of Israeli government wrongdoing.

Ian Hislop did not have to write an editorial in Private Eye. He clearly felt that he needed to place his conversation in the public domain.

He was right to do so for one reason in particular. Julian Assange has yet to explain his relationship with the Holocaust denier and antisemite Israel Shamir. The statement issued by WikiLeaks that said it dealt with Mr Shamir as it would have done with any other journalist does not wash. WikiLeaks should not have dealt with him at all. Mr Shamir is not like any other journalist. Index on Censorship, which has been a consistent supporter of WikiLeaks on freedom of speech principles, has failed to secure the reassurances it has asked for on the Israel Shamir issue. This is especially worrying for dissidents in Belarus, where Mr Shamir is alleged to have passed cables to the authoritarian president Alexander Lukashenko.

The Shamir issue is becoming increasingly difficult to explain away. Until he does, Julian Assange cannot ask to be taken seriously as a campaigner for freedom. If you choose to tolerate or defend a nasty antisemite, it is only a matter of time before people begin to wonder whether you are a nasty antisemite yourself.