What a brilliant piece of stagecraft it was. Boris Johnson mounted the platform at the Conservative party conference, set out his script, then peered around and demanded: "Where is Dave?" In the seventh row of the stalls, was the answer. The prime minister was laughing so hard it must have hurt, when his dearest private wish can only be that a divine thunderbolt reduces his old mate the mayor of London to a pile of cinders.

One of the worst things that can befall any public performer is to do a turn behind somebody who is funnier and more popular than he is. Boris Johnson is not merely much better-loved than the prime minister, he is the most popular politician in Britain, with a plus-30 positive rating in an opinion poll last weekend, against Cameron's minus-21. The public, and especially the young, do not just like the guy; they love him.

He was greeted in Birmingham by rock star style acclaim. At a moment when the Conservative faithful recognise that their party is on course to lose the next general election to a rabble of leftwing zombies, they see Boris as the fat white hope, Joan of Arc, Francis of Assisi, El Cid, Superman, Indiana Jones.

His speech to conference reflected all his wit, brilliance and showmanship. He is an authentic star, who lights up every room he enters. He makes people laugh and feel good. He sings a song that the British people – who currently despise almost every other politician in the pack – will crowd any venue to hear.

Why should he not be prime minister? Why should Boris not be the man to leap forward and save party and country from the dark forces? My own answer is that if the mayor of London is the answer, there is something desperately wrong with the question.

If the day ever comes that Boris Johnson becomes tenant of Downing Street, I shall be among those packing my bags for a new life in Buenos Aires or suchlike, because it means that Britain has abandoned its last pretensions to be a serious country.

I have known the mayor more than 20 years. He worked for me as EU correspondent of the Daily Telegraph and then as a columnist when I was the paper's editor, and I have seen plenty of him since. He is a magnificent journalist and showman. He proved himself the perfect maitre d' for the London Olympics. But few maitre d's are fit to cook the dinner.

Most politicians are ambitious and ruthless, but Boris is a gold medal egomaniac. I would not trust him with my wife nor – from painful experience – my wallet. It is unnecessary to take any moral view about his almost crazed infidelities, but it is hard to believe that any man so conspicuously incapable of controlling his own libido is fit to be trusted with controlling the country.

His chaotic public persona is not an act – he is, indeed, manically disorganised about everything except his own image management. He is also a far more ruthless, and frankly nastier, figure than the public appreciates.

When one of his many sexual affairs was exposed and much trumpeted in the headlines, he telephoned a friend of mine who was then running one of Britain's largest media organisations. "It's utterly disgraceful what your reporters are doing on-screen about my private life," spluttered Boris. "It's time you realised that I know all about your private life. If your organisation goes on reporting my affairs like this, you'll be reading all about yours in the Spectator [the magazine he then edited]."

My friend responded: "Stop a minute, Boris, and think about what you just said. There is a word for it, and it is not a pretty one – 'blackmail'." Johnson waffled away, muttering that he had never really meant it. But he is much given to making threats, bearing grudges and behaving with malice aforethought.

I would not take Boris's word about whether it is Monday or Tuesday. I feel a twinge of regret for speaking so harshly, because I am as susceptible as most of the British people to Johnson's brilliant, warm, funny public persona.

But this is not a man who aspires only to fill his familiar slot on the BBC's Have I Got News For You?. Forget all the denials – Boris yearns with a mad hunger to become prime minister. Some Tory MPs are so panicked by their current standing in the opinion polls that they have persuaded themselves that London's mayor is the future.

On the basis of what, some of us would ask. Boris bikes on London's streets? The peerless jokes and bonhomie and TV wizardry? Testimonials from ex-lovers who found him amusing in bed? A friend said to me not long ago: "When will you understand that the reason the young are potty about Boris is precisely because he is not serious, because he treats the whole business of politics as a bit of a lark." This is true.

But his lovability, or even bonkability, is not the point. One of our biggest problems as a society is that we have become obsessed by the X Factor culture. We no longer look for dignity, gravitas, decency or seriousness of purpose in our leaders in any field. We demand only stardust, a jolly turn in front of Simon Cowell or on Strictly Come Dancing.

I knew quite a few of the generation of British politicians who started their careers in 1945 – the likes of Roy Jenkins, Denis Healey, Edward Heath, Enoch Powell and Iain Macleod. The common denominator among them all, whatever their party, was that they entered politics passionately believing they could change things. They were serious people. It does not matter whether they were wrong or right – almost all of them had real beliefs.

Today most aspirant politicians of every party have not a personal conviction between them. They merely want to sit at the top table, enjoy power, bask in the red boxes and chauffeur-driven cars, then quit to get as rich as Tony Blair.

Boris Johnson was at the Tory conference on Tuesday for one purpose only – the exaltation of himself. This does not much matter when he is only mayor of London, but would make him a wretched prime minister. He is not a man to believe in, to trust or respect, save as a superlative exhibitionist. He is bereft of judgment, loyalty and discretion. Only in the star-crazed, frivolous Britain of the 21st century could such a man have risen so high, and he is utterly unfit to go higher still.

© Daily Mail 2012