Four years ago I committed the only political act of which I am thoroughly ashamed. I shudder with self-loathing when I look back at how blinkered and wrongheaded I was. I voted for Ken Livingstone to remain Mayor of London.

There. I’ve admitted it. I can offer you excuses; I can give you reasons. But none of them really matter, because the fact is that I voted to keep in office a man who besmirches his office simply by occupying it.

So when I read David Aaronovitch in The Times last week, the full horror of my actions came flooding back. “Ken,” he wrote, “wrong on all the things that don’t matter in a London mayor, has been right on almost all the things that do.” The mayor, he concluded, should be re-elected on May 1.

David, I understand where you are coming from. I’ve been there myself. But I’m surprised – to put it mildly – that you haven’t realised how deeply, profoundly wrong we both were to vote for Ken.

Four years ago I decided that there was one overriding issue in the election: the congestion charge. Steve Norris opposed it. Ken Livingstone had introduced it. QED, as a supporter of the charge and of the mayor’s emphasis on renewing public transport, I should vote for Ken.

How could I have been so blinkered? It’s obvious to me now, after four more years of Ken Livingstone, that such a calculation is positively idiotic. The things that you say don’t matter in a London mayor – such as the invitations to the Muslim cleric Sheikh Yusuf al-Qaradawi, who supports suicide bombing and the stoning of homosexuals; the hero-worshipping of tyrants such as Fidel Castro; the shady deals with Hugo Chávez; the smear campaigns against opponents such as Trevor Phillips; and the City Hall fiefdom of incompetents and leeches on the taxpayer – do matter. A lot. They go to the heart of what it means to be mayor of a cosmopolitan, vibrant city.

Ken Livingstone has behaved as if conducting a masterclass in how not to behave as a mayor, not least in his apparent readiness to insult anyone Jewish who questions him, such as when he compared a Jewish reporter with a concentration camp guard and said of David and Simon Reuben – two Indian-born British businessmen – that “they can always go back [to Iran] and see if they can do better under the Ayatollahs”.

A vote for Ken is not simply a vote for better transport. It is, and can only be, an endorsement of Ken Livingstone in all his guises.

In choosing a buffoon such as Boris Johnson as its candidate, the Conservative Party has revealed its own contempt for the electorate. But like it or not, the next mayor will be one of these two. And the idea of re-electing a man who defends clerics who want to stone homosexuals surely means that there is only one option.

Times Recommends

• Graveyard of empires could claim more careers
• Would you live on the wrong side of the Berlin Wall?
• At last we’re getting our heads out of the sand