Days before the royal wedding, a British constitutional debate has taken an interesting turn.

Britain's Deputy Prime Minister Nick Clegg, leader of the Liberal Democratic Party, has questioned male primogeniture - the rule that the crown passes to the monarch's first-born son ahead of an older sister.

The rule is indefensible today. John Key can confidently declare New Zealand's support for allowing the first born of either sex to succeed, and it would be surprising if any of the Queen's 16 Commonwealth realms disagreed.

Male succession has nothing to recommend it historically. England's most prosperous and celebrated eras carry the name of queens - Elizabeth I and Victoria. Elizabeth II's near-60 year reign has been as notable in its own way. If longevity is the secret of royal success, women are naturally blessed.

But if the rules of succession are to be adapted to contemporary values, some may ask, why stop at sexual equality? If male primogeniture is no longer acceptable, what about primogeniture itself? Where is the fairness in favouring the first born?

Britain might have avoided the agony of an abdication in the 1930s if the previous King had been able to anoint the son more likely to serve the nationalinterest.

But then, if a choice is to become possible, should the monarch be the one to make it, and should the candidates be restricted to the monarch's children? To tamper with the rules of succession soon leads to questions that challenge the idea of the head of state being a privilege of birth. Few want to go that far, particularly on the eve of an heir's wedding.

Prince William and the soon-to-be Princess Catherine offer a brighter future for the monarchy.

In questioning male succession, the Liberal Democrat leader did not challenge the institution. If the royal couple's first child was a daughter, he said, "I think most people would think it perfectly fair and normal that she would eventually become queen of our country".

Mr Clegg, whose party is arguing for a change to Britain's voting system at a referendum on May 8, probably raised the subject in the hope voters might warm to the possibility of constitutional change generally.

They are being offered an Australian electoral system that Britain calls alternative voting.

After a good start, the case for reform is struggling. Recent polls suggest the referendum will confirm the status quo. The debate has divided the coalition and the opposition. Some former Labour ministers are siding with Prime Minister David Cameron, Labour leader Ed Miliband is with the Liberal Democrats.

The alternative vote would not change much. It is not proportional representation - it would eliminate minor parties on the first count and allocate their voters' next preference to produce a winner with more than 50 per cent.

The two main parties would be likely to win electorates on second preferences as they do in Australia.

The subject shows Britain's conservatism on constitutional issues, including the monarchy. The only concern heard in Britain about broadening the succession rule is that it might encourage republican tendencies in Australia.

When the world watches the royal wedding on Friday, it will see a couple capable of modernising the monarchy and keeping it alive for their children's generation, whatever the sex of successors.