When historians come to chart the fate of the monarchy at the end of the 20th century, Prince Charles’s diary musings will be a central text, reflecting the extraordinary confusion surrounding, and within, the Royal Family, over what exactly it is for.

The Prince’s journal portrays a man torn between the Divine Right of Kings to travel first class, and the instincts of a would-be politician for the people; a man who accuses the Government of being out of touch, while bitterly bemoaning the loss of his “dear yacht” Britannia; who regards himself as a “dissident”, but insists that his dissenting views be circulated only among people who may be counted on to agree with them.

The Prince believes that by addressing modern issues in a forthright way he is adapting the institution of the monarchy. Instead, he risks damaging it deeply, and perhaps fatally.

I rather like some of his reflections on the handover of Hong Kong. An “appalling group of old waxworks” is a fine description of the Chinese gerontocracy. I was unaware that artificial wind machines were used to give the flags a bogus flutter at that grim ceremony marked, as he rightly puts it, by “exasperated sadness”. His description of trying “to make a speech underwater”, as his text turns to pulp in a sudden downpour, is really quite funny. In another life, Prince Charles would have made a decent newspaper journalist.

He is entitled to his views; some of these are quite interesting, while others are quite barmy. What he is not entitled to do is use his position to try to turn his opinions into policy. In seeking to influence and comment on politics, the Prince follows an ancient, but anachronistic, royal tradition. Throughout history, ambitious royals have sought to play a political role, often successfully, pressurising governments and ministers in one direction or another, and even setting themselves up as an alternative political focus, as the Prince seems to see himself.

The Queen's great achievement was to realise that, in postwar Britain, the monarchy could no longer afford an overt and personal political role. The notion of political power exercised through the accident of birth was dying fast, and the monarchy would die too if it did not adjust. Deference is still owed to monarchy, but to the institution, not necessarily to the person of the monarch, let alone his or her opinions. With great wisdom, and doubtless some frustration, the Queen has buttoned her lip for 54 years.

The Prince, by contrast, appears to believe that his views deserve attention and implementation because of who he is. That belief — equating privilege with power — runs through every word of his journals. He seems to believe that because he will one day rule, he also has some mandate to govern, or at least to influence government.

Flying to Hong Kong, the Prince griped that mere politicians such as Robin Cook and Paddy Ashdown were “comfortably ensconced in first class immediately below us”, while he and his retinue were slumming it in club class.

But in truth, he thinks and behaves like a politician; he should therefore renounce his royal status, accept ordinary citizenship and become a politician. The man who would be King would make a rather better MP.

Chatting with Tony Blair in Hong Kong, Prince Charles spoke of “the identity problem that Britain has . . . an inability to know what to do next”. He was plainly talking about himself. And what better way to demonstrate his own identity, and push the causes he feels passionately about, than by obtaining a genuine mandate through elected office? Then he would no longer have to leak his opinions through disloyal servants or secretly lobby exasperated ministers.

There are precedents, both literary and actual. In Shaw’s play, The Apple Cart, published in 1929, King Magnus refuses to conform to his constitutional role and terrifies his ministers by threatening to abdicate and stand for election in the Windsor constituency.

In 2001 Simeon II of Bulgaria, the country's last Tsar, returned to his homeland and formed a new political party, the National Movement for Simeon II, dedicated to “reforms and political integrity”. The party went on to win half the 240 seats in the Bulgarian Parliament and Simeon served as Prime Minister until 2005, the only monarch in history to regain political power through democratic election to a different office.

I am not sure Britain is ready yet for the National Movement for Charles III, dedicated to “organic farming and talking to pot plants”, but I can see him as Mr Charles Windsor, the Honourable Member for Windsor. Given his talent for scandal, he might do best in the Liberal Democratic Party. He he has all the attributes of a solid backbench MP: a jolly and presentable wife, a set of causes, solid self-belief, a social conscience, deep patriotism and some harmless eccentricities.

He also has a taste for upsetting the apple cart and fomenting controversy, ideal in a politician, positively dangerous in a modern prince. It is plain he will never consent to the role laid out for Queen Victoria 150 years ago: “Nothing but a mandarin figure which has to nod its head in assent, or shake it in denial, as her minister pleases.” Victoria, of course, did nothing of the sort; but she could wield power through the privilege of birth in a way that the Prince could do today only if he earned it. The very neutrality of the Crown is what ensures its continuance.

Prince Charles has a choice. He can either remain in the first-class seats and keep his counsel; or he can join the rest of us in goat class, and speak his mind.

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