Vanity is no respecter of persons. The deputy prime minister, when asked on radio only a couple of weeks ago which actor he would like to star as John Prescott in a film of his life, chose Marlon Brando. You have to laugh. The thought that jowly Two Jags harbours an idea of himself incarnate as one of the most beautiful men and most celebrated serious actors of the 20th century is just too good to be true. He went on to say in the same interview: “I like to come out walking like John Wayne.”

What a difference a few days make. Somehow I do not think that Prescott is going to come walking out of the lurid revelations of last week like John Wayne, that epitome of the strong, spare, quiet and righteous hero of western morality tales. On the contrary, Two Jags staggers out of last week’s news like a vain, fat stumblebum, a sexually incontinent slob, tolerated by his peers only for political advantage, as a token old-leftist. Perhaps that is a role that Brando might have wished to play; I rather doubt it. It lacks redeeming features, other than the loyalty of a tolerant wife.

However, it has been wonderful low comedy; I have howled with laughter at the photographs in the tabloids of Prescott’s clumsy canoodlings with a civil servant, the allegedly insatiable 43-year-old assistant private secretary, Tracey Temple. On Wednesday Prescott confessed to an affair with her, during which he had used his elegant official residences (Three Pads) for “trysts”, one of those words like “denizens” that only appear in the tabloids when they are tut-tutting. Then it emerged that Tracey had gone to that awesome scourge of the famous, Max Clifford, to sell her story.

The voyeurs among us have been able to feast our eyes on photographs of Prescott at an office Christmas party, employees and underlings around him, beaming at the camera and holding his mistress up in the air with one of her solid legs round his very solid neck, or of treacherous Tracey unbuttoning his shirt while he gazes squiffily at the camera, or of the pair of them apparently grappling each other to the ground. There were others, too, of the lovers publicly attending a national memorial service together, and of Tracey solemnly escorting Prezza’s wife to the opening of parliament.

Two Jags is the man, let’s not forget, who hurled stones of righteous anger at John Major’s government. “The most desperate, despicable, seedy, grubby, hopeless, lying, hideously incompetent bunch of third rate, double-dealing disasters this great nation has ever seen,” he raged in 1994, with most uncharacteristic articulacy. “For too many Tories, morality means not getting caught,” he intoned in 1996. “Morality is measured in more than just money. It’s about right and wrong. We are a party of principle . . . we’ve had enough lies, enough sleaze.” Oh dear.

However, hilarious this may be, I would normally feel obliged to say that it is a private matter. We shouldn’t really be reading about it. What people do in private should remain private. Privacy matters, not least for people in public life. This is the line the prime minister has taken. So has Gordon Brown. And the first response of the BBC, The Guardian and most Conservatives, was primly to underplay the story.

This was also the view the Cambridge Union came to last Thursday. I had been invited to defend the motion that “this house believes that those in the public eye deserve a private life”. Despite a little quibbling about the word “deserve”, since most of us were doubtful that politicians and footballers are necessarily very deserving, our side won comfortably. This was despite the fact that the obvious arguments for extreme freedom of the press were well put for the opposition by a photojournalist and an American investigative reporter.

One of the only good reasons, I believe, for violating a person’s privacy is that what he does in private might suggest that he is unfit for public office. If so, he loses his claims to privacy. Otherwise, what he or she does only concerns those most intimately involved; and we hardly need any squalid revelations about Prescott’s grace-and-favour rumpy-pumpy to know that he is unfit for his office — incompetent, incoherent and emotionally incontinent, a bluffing, blustering, bullying thug. We knew that anyway. This scandal demonstrates little that wasn’t public knowledge.

Adultery is so common among public people that it is almost normal. If extramarital sex were a disqualification, the numbers of people in public life would dwindle almost to nothing. Politicians, for some reason, seem to be rather oversexed.

Besides, it has never been clear to me that committing adultery is proof in itself that the adulterer is unfit for office, or indeed unfit for marriage. Jack Kennedy, idolised by millions, made Bill Clinton look like a homebody by comparison; JFK felt, apparently, that he needed sex three times a day, otherwise he got a headache. These are murky waters. People’s sex drive varies; people’s marriages and private compromises vary. For all I know, Mrs Prescott may have known about her husband’s goatish disposition and decided to put up with it.

However, there is something different about the question of Prescott’s privacy in this scandal: he violated it himself repeatedly. Thereby he lost any right to it. Prescott flaunted his mistress, publicly, in the most vulgar and intrusive way, in front of people over whom he had power, happily posing for photographs.

No discreet secret love nest for Prezza. He was showing off shamelessly and in the process he abused his staff and his civil servants, indifferent to their moral feelings and embarrassing them with his wife by making them complicit in his behaviour.

He abused the privileges of office by taking his mistress to his official residences and he abused the dignity of his office by behaving like a geriatric lout. It may be that in his entanglements with treacherous Tracey he has broken some civil service rules; but in any case, by his coarse disrespect for public propriety and public manners, and his total indifference to the boundaries between public and private, he has brought his office and his government into disrepute.

Prescott has forfeited any right to privacy and he has invited disgrace and dismissal — and all because he couldn’t help showing off. Vanity is no respecter of persons.

Times Recommends

• Treat Africans as part of the human jigsaw
• Too bitter a pill for me to swallow
• This Bill is a panic measure in a tarnished age