The purpose of the British Royal Family is procreation; its prime duty is to produce at least one heir to the throne. Each heir has to provide a child that will guarantee the survival of a monarchy that began with Athelstan, the first king of all-England in 926.

The baptism of Prince George is the second of his public appearances on the road to his coronation as George VII. Royal job done. Or is it?

In spite of his celebrity parents, Prince George’s chances of being king are not as high as most appear to think even though the monarchy is more popular than it has ever been during this sovereign’s reign.

A ComRes opinion poll in July showed that 53 per cent of those questioned believed Britain would be worse off without royalty. Even a committed republican rump thought that even in a hundred years time Britain would still be a monarchy. The evidence is, at first sight, overwhelming.

The Queen remains the most admired member of the Royal Family although 18 to 24 year olds say the Cambridges are the most popular; 38 per cent want them to have the throne when the Queen dies. Ostensibly, the British monarchy is safe.

So, at the risk of disturbing the grating of Traitors' Gate and risking a spell enduring HM’s displeasure, I would argue that royalty should not be betting the jewels on making it as far as they think they will.

Interpretations of polling often overlook the considerable difference between monarchy and the reigning monarch. Voting for a monarchy is supporting an institution. Voting for the sovereign is quite a different X-factor. The sovereign is a celebrity and celebrity is everything. The Queen is probably among the top ten brand images in the world. Oddly and in spite of general perceptions, the world has only a superficial view of the Queen unlike her children and grandchildren who are gossip fodder – two open adulterers, two sons of a serial adulterer. This the very stuff of British monarchy history. The Queen is different.

When she succeeded her father, George VI, the public knew little about the then 26 year-old Elizabeth. Even her nanny, Marion Crawford who claimed to tell all in 1950, did not give the public much insight to their future monarch. Throughout her reign, the Queen kept her distance from her subjects. Few will remember more than two things their monarch said during six decades. Last year, at the top of her royal game, Elizabeth II was seen as a kindly figure at her Diamond Jubilee and “so good for her age” as they say in the better Windsor care homes.

Thus the monarchy is safe in her hands. Good Scottish stock is reliable. Her heir is in a quite different position. The media and the Prince of Wales himself have told all – or almost all. The polling numbers tell us that the public is clearly divided over Charles. When (if) he becomes king, Charles will not expect the public expressions of loyalty and understanding shown to his mother. The institution will suffer.

But it is changes within the other great institutions that will bring questions about the monarchy, rather than the monarch.

For example, the Church of England is the state-controlled and declining church. It will be disestablished from Parliamentary control. The monarch is the Supreme Governor of the Church and still, Defender of the Faith. Without the church as an instrument of the state, the sovereign would lose – as George III observed when refusing Pitt the Younger his Catholic Emancipation Bill – a central commitment of his coronation oath.

Next, House of Lords reform will be far more dramatic than now imagined. The real Lords question will be put: why have a second chamber at all? Furthermore, whatever the reform, government will not see a role for a sovereign to do the My Government Will speech.

There remains the role of the monarch and Commonwealth. Once the Queen is dead every political indicator says that Canada and Australia will vote for republicanism. Others will follow. The Commonwealth will easily survive. The monarch’s symbolic role within that association of a quarter of the world states will be reduced.

There is too the role of the royal family. Public opinion shows little support for the wider family. Some, especially the princesses Beatrice and Eugenie are even figures of fun and devalue the solemnity of monarchy by being sixth and seventh in line to the throne.

But have not the Cambridges rescued the monarchy’s image? There is no evidence that William and Catherine have reconnected the royals with the people. The Cambridges are very much celebrities appealing to a younger generation. But there is only anecdotal evidence that they have improved the image of the monarchy. Clearly, the Queen has done that and there is nothing to suggest that the Prince of Wales will sustain that relationship. After his coronation, the polling history of the royals will be re-written.

After Charles will come the universally popular William and Catherine. Young, smiling, connecting as young parents openly preferring a night in with a fish supper. That’s a now image. Given the longevity of the British royals, they will both be well into middle age by the time William is crowned and the pollsters will have quite different questions to ask.

On similar actuarial evidence, George could be well into his 60s before crowning – certainly 60 years from now. Here is the earth in the debate over royalty’s future.

Given the pace of change we have witnessed during the first two decades of the twenty-first century we can only wonder at what might be the global influences on British society from Europe and beyond, economic transitions, social and ethnic migrations plus radical institutional reform during the out years of the next six decades.

Fifteen years ago no one outside Stanford University had heard of Google. Today it is a verb. That’s how fast the accepted institutions and ways are changing. For the past 300 years, institutions that supported or needed monarchy evolved. Today they change altogether, even disappear. Few any longer accept their values.

When the institutions are dissolved so the monarchy will lose its final and perhaps greatest role: the monarch, since Athelstan, has more than anything defined the identity of the nation.

During the next sixty years that national identity and what matters to it will undergo the most radical change of all. The monarchy will simply go out on the ebb of that identity change. When it does, the tide will not turn in its favour. The monarchy will have served its purpose and there will be no crown, even an hollow one, for George to be impatient to wear.

Christopher Lee is a former BBC Royal Correspondent and the author of the new book Monarchy, Past Present and... Future? published by Bene Factum