On social justice, environmental safety, human rights, countryside issues - Charles Windsor has no shortage of passionate beliefs.

And, as a private citizen, he is as entitled as anyone to do and say what he likes. If only he wasn't going to be king one day.

It is one of those peculiarities of the unwritten British constitution that important matters of state - such as the role of the monarch - are largely governed by convention.

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The classic definition of the monarch's role was written by 19th Century writer Walter Bagehot: "The Sovereign has, under a constitutional monarch such as ours, three rights - the right to be consulted, the right to encourage and the right to warn."

But apart from that, the public role is to do whatever the prime minister tells him or her.

Which is why Charles may have to bite his tongue when he has the ultimate rebranding and becomes King Charles III.

Not the job

For while the social and education work The Prince's Trust is widely admired, this week by the Queen herself, some of his other views and activities may just not fit the job.

Professor John Alder of Newcastle University said Prince Charles was a private citizen, so he could do what he likes. But as the monarch he would have to keep out of political issues.

Charles missed a banquet with President Jiang due to a personal engagement

"The only political role of the monarch is A, in private, and B, to intervene if the government collapses," he said.

It will certainly mean no more criticising bodies such as the National Gallery for its choice of architects.

And no more publicly questioning the safety of genetically-modified food.

And while personally he may well have sympathy with the Dalia Lama and the people of Tibet, it will have to remain a private view.

Attendance at state banquets will, of course, be obligatory. (Some people thought his decision not to go to a banquet with Chinese President Jiang Zemin was a snub, although this was denied.)

Prince William went hunting with his father this week

And if the PM has stated he is going to ban fox hunting, it might not be politic to go out hunting with press photographers in tow.

Having prepared himself for being king for most of his life, Prince Charles will be completely aware of what will be appropriate for him to do.

Harold Brooks-Baker, publisher of Burke's Peerage recently said he thought the prince's failure to attend the banquet with President Jiang was a sign that Charles would do things his way.

"This is a clear indication that should Prince Charles be king, he will not be a puppet king," he said.

But historian Hugo Vickers said the prince will know he has far more freedom now than he will have as sovereign. So if he wants to make his contribution, now is the time to do it.