A political prince cannot make a constititutional king

Prince William, second in line to the throne, was asked on Saturday what he was going to do after he graduates from St Andrews next year. "I've not made any serious plans yet, but the armed forces would be the best move at the moment," he said. He was taking part in some PR stunt at Home Farm, Highgrove, where he was photographed helping out his father, the Prince of Wales, with feeding pigs, driving tractors and casting an eye over a herd of cows.

At first glance, this tiny episode appears as inconsequential as 99 per cent of the coverage that newspapers give to the Royal Family. But if it matters how Prince Charles employs himself as heir to the throne, then what the next-in-line does is also significant.

For, in constitutional terms, Prince Charles has been following a dangerous course. He comments publicly and campaigns privately on a range of political issues in a way which is forbidden to the monarch. Indeed, seeing this, the historian Tristram Hunt has asked in this month's issue of Prospect magazine whether the future King Charles III would willingly sign legislation banning fox-hunting or extending stem cell research, and whether he would continue to snub the leaders of a major power such as China when they visit London, as he has done in the past.

Prince Charles has remarked that he happens to be "one of those people who feel very strongly and deeply about things". In a speech given in 2002, he confessed: "I have come to realise that my entire life so far has been motivated by a desire to heal - to heal the dismembered landscape and the poisoned soil; the cruelly shattered townscape, where harmony has been replaced by cacophony; to heal the divisions between intuitive and rational thought, between mind and body, and soul, so that the temple of our humanity can once again be lit by a sacred flame."

It is difficult to know what to make of these words. They combine mysticism - "sacred flame" - with a list of impossible tasks, such as healing the divisions between intuitive and rational thought, together with a longing for some golden age in which townscapes were harmonious and not confusion and muddle as the Prince seems to think they are today.

In furtherance of his views, Prince Charles has embarked upon a range of activities, some wholly laudable and others that raise questions of constitutional propriety. The Prince's charitable work is exceptionally good. He has gone much further than merely lending his name to good causes. Indeed some of the Prince's initiatives are close to the thinking behind government policies such as the Chancellor of the Exchequer's New Deal scheme. They promote urban regeneration and stronger communities. Prince Charles may call this work "healing" but I prefer to think of it as what a "social entrepreneur" does, which is to bring ideas, people and funds together not for profit but for social improvement.

Strictly speaking, Prince Charles should stop right there. If this seems a strange thing to say, it is because the constitutional arrangements we have in this country are a very odd way of determining rights and obligations. Almost every other nation writes down the relevant rules and calls it a constitution. But we rely upon conventions as well as statutes, on the unwritten and well as the written. There is no job description, for instance, for being Prince of Wales. But as Lloyd George reminded the future Edward VIII: "If you are one day to be a constitutional monarch, you must first be a constitutional Prince of Wales."

In other words, successive heirs to the throne have to deduce from the monarch's role the extent to which their own activities should be circumscribed And here the relevant fact is that the monarch, which the Prince will one day become, can act only on the advice of ministers. Even the speech which the Queen reads out at a state banquet is written for her in Whitehall. We do not permit the monarch to play any role in policy formation.

While he waits, however, Prince Charles attempts to influence government actions by means which he knows he won't be able to use when he ascends the throne. He addresses five main subjects, agriculture, architecture, the constitution, education and science. He prepares policies with the help of sympathetic experts; he gives speeches and he regularly writes to ministers about his concerns. In short he acts like a pressure group except that he has more resources and better access to power than most. And ministers take notice.

Of course we cannot be too critical. Prince Charles is trying to do good in his own way, even though he will have to stop when he becomes king. Or will he? That is the worry. He has not followed Lloyd George's injunction.

Actually he is a mildly unconstitutional Prince of Wales. And this in turn is why it matters what plans Prince William makes. To make a career in the armed forces as he suggests is the traditional solution to the problem. It is a good first step. And in due course he will be able to make up his own mind about his father's example.