Off with her talking head!

If there is one person who should be looking closely at Lord Mackay's new Divorce Bill - and maybe also his Domestic Violence Bill, since it covers mental cruelty - it is the Prince of Wales. For if ever there was someone who needs to get shot of a spouse in the speediest and most painless way possible, it is the heir to the throne.

It really does not matter much what his estranged wife says in her interview on Panorama, which will be broadcast tonight. The mere fact that the Princess of Wales chose to organise this public appearance without any consultation with Her Majesty the Queen is, by itself, an argument for divorce.

This is a programme that will be viewed by many millions of viewers, not only in this country but throughout the Commonwealth, in which the Princess apparently gives her opinions on matters as sensitive as her relationship with Prince Charles and the next-in-line to the throne, their son William. This, as she well knows, is serious stuff. To discuss such matters on prime-time TV without consultation with the Queen is more than a breach of protocol. It demonstrates her unfitness to remain formally linked to the Royal Family.

Of course, this is only the latest of many deplorable lapses on Princess Diana's part. To be sure, not all of this publicity has been solicited by her. She was not to blame when transcripts were published of her telephone conversations with James Hewitt. Nor was she responsible for the snapshots of her pumping iron in one of the London gymnasiums, where she spends such a large amount of her time.

Nevertheless, Lord Wakeham has a point when he hints that Diana is far from being simply an innocent victim of paparazzi and telephone-tappers. It was not perhaps wise of him to write an article of the sort which appeared in yesterday's Mail on Sunday - apparently in pique that the Princess rejected his advice at a private dinner last month. As chairman of the Press Complaints Commission, it is not his job to review television programmes before they have even been broadcast. But he has said what needed saying.

Some time ago, the Princess of Wales struck what can only be described as a Faustian pact with the British media. She began to manipulate the press in order to further her own ends in what she has come to see as a Manichaean struggle between herself and the royal establishment.

The most glaring example of this was the book by Andrew Morton, Diana, Her True Story, supposedly based on the testimony of her "friends". But since then she has edged ever closer to direct communication with journalists. Photographs of her entering a car with the Daily Mail's royal correspondent spoke volumes. With her Ray-Bans and baseball cap, the Princess was revealed as the Royal Family's answer to Deep Throat. Tonight's interview is merely the culmination of a long process of media manipulation.

What the Princess seems not to realise is the true nature of the Faustus bargain. Somewhere along the line she sold her soul and the price of all that publicity is going to have to be paid sooner or later. For the fact is that a royal divorce is now an urgent necessity. There are, of course, many who do not share this view. Some are covert republicans. Others, like Auberon Waugh, see the Princess as a breath of fresh air for the monarchy. His argument seems to be that the sooner the Queen and Prince Philip appear in Ray-Bans and baseball caps, the better. But most opponents of divorce are more conservative. Because they wish to promote "family values" at a time when the institution of marriage is in decline, they dislike the idea of a royal divorce, arguing that the Royal Family ought to "set an example". They worry about the effect a divorce would have on the royal couple's sons (a card we can expect Diana to play tonight).

But this is sentimental twaddle. The point about the Royal Family is that they are not like everyone else; their example is not applicable to our lives. And the crucial way they are not like everyone else is this: their persons come a definite second to the institution of the monarchy which they, during their lives, personify.

Some people evidently need reminding: the institution of the monarchy matters. It has been one of the peculiarities of this country that while most other European countries have lurched from monarchy to republic to dictatorship at one time or another since 1789, Britain has not - having tried and rejected the alternatives three centuries ago.

You may think the Queen looks silly in ermine and a tiara. You may think she has more big houses than is good for her. But that is to see only the surface of the monarchy. Its deeper constitutional efficiency lies in the way it has come to embody national (and, indeed, imperial or Commonwealth) unity by rising above party politics. Can those who despise the monarchy maintain they would respect a democratically elected head of state - say Bill Clinton or Boris Yeltsin - more?

Now, it is not easy for a loyal subject to criticise a reigning monarch. But I am sorry, Ma'am: the choice of Diana Spencer as your son's wife was a ghastly blunder. True, you were not to know that she would grow up to be a narcissistic harpie - through her family history should have warned you. But would it not have been wiser to have married Charles to the daughter of one of the more innocuous continental houses? The historic reading strongly suggests that marrying non-royals is hazardous, even (or perhaps especially) when they are connected to reasonably well established aristocratic families. To endure the self-abnegation that comes with royal status, you have to be born and bred to it.

Perhaps the Queen's mistake was simply this: Diana was not another Wallace Simpson - a divorcee whose relationship with the Queen's uncle led to his abdication - whereas Camilla Parker Bowles might have been, had Charles married her for love. But, alas, Diana is turning out to be every bit as lethal to Charles's hopes of a happy and glorious reign as Mrs Simpson was to Edward VIII's.

But what of the argument that the future King of England cannot divorce his future Queen? This is just plain nonsense. The Church of England was invented in the 1530s by Henry VIII for the express purpose of divorcing a Queen - poor old Catherine of Aragon, who had failed to provide Henry with a male heir. Diana, of course, has succeeded where Catherine failed. But that does not mean Charles should not divorce her if she threatens to bring the Crown into disrepute.

As Lord Blake reminded us last week, there is a good precedent for doing so: when George IV excluded his Queen, Caroline of Brunswick, from his coronation. Like Charles, George was no saint. Like Diana, Caroline was hugely popular. But in the end the monarchy survived the showdown and Caroline died in disreputable obscurity.

Something along those lines seems to be the best possible fate which could befall Diana following a divorce: retirement to Grand Cayman, where she can continue to live out her film star fantasies in the company of married sportsmen and photographers from Hello! magazine - but no longer as a member of the Royal Family with official privileges and responsibilities, though it may be prudent to keep her on a small allowance conditional upon good behaviour.

Of course, she will fight every inch of the way, like Caroline. But for the Queen there is now no alternative. And the sooner the divorce proceedings are initiated the better. Has Diana considered what might happen if this were to blow up under a Labour government, with all that party's republican fellow-travellers in power?

Diana should count herself lucky as the royal lawyers present her with their terms: 400 years ago, she might well have faced a rather sharper exit - on the block of a Tudor scaffold.

The writer is a Fellow of Jesus College, Oxford, where he teaches history.