One of the weird things about the monarchy is that it makes births, birthdays, deaths and marriages matters of constitutional importance.

By linking public office to private family life the Crown ensures that the Queen’s 90^th birthday isn’t just a moment to be enjoyed by Elizabeth Windsor and her family, but a cause for wider public debate. What should simply be a private affair is a public reminder of an imminent succession.

The palace PR machine, with the enthusiastic support of the BBC, exploits these personal moments to promote the royal brand. We’ve seen it this week with a carefully managed interview with Prince William and bizarre claims about how the Queen has managed to live so long.

For the rest of us these moments are an opportunity to look back on the Queen’s record in office, look forward to what’s coming and to think about what the public celebrations say about this archaic and unaccountable institution.

I talk about a record in office – rather than a reign – because that’s what it is. The Queen is head of state, she has a job to do and a term in office that extends from her father’s death to her own.  She has a record for which she cannot be held to account, but by which we can judge her performance.

A lot of people instinctively say she’s ‘never put a foot wrong’, building the myth that the Queen is to be trusted in contrast with the scandal ridden politicians. Yet if we think the Queen has been a great head of state then we really are setting the bar very low.

A head of state in a parliamentary democracy should be independent of the government, able to perform some meaningful constitutional function and should be absolutely above board on questions of probity, accountability and transparency.

In a parliamentary system where the prime minister runs the government the head of state should be able to represent the nation, speak for us and to us at times of national celebration and crisis. You might at least expect to be able to remember some great speeches or eloquent interventions in national life.

The Queen’s record, alas, is one of silence, obedience to the prime minister, dogged determination to defend her own interests, secrecy, abuse of public funds and resistance to any serious scrutiny.

Constitutionally the Queen does as she is told by the PM. You may think that’s the right thing to do, but that’s because we’ve been brought up to believe that the rules of a constitutional monarchy are the rules of parliamentary democracy. In a parliamentary democracy there is supposed to a plurality of power, checks and balances and limits to the authority of executive, legislature and head of state.

The head of state should be able to guard the constitution, to act as a referee in the political process. It’s a role some claim the Queen plays, but as referee she will only ever follow the orders of one of the team captains, the leader of the governing party.

This renders her silent unless pressed upon by the government to make some guarded dog-whistle comments on matters such as Scottish independence or the EU.

Powerless to play a meaningful role in public life, the Queen instead puts a lot of energy into defending her own interests. As Joan Smith points out this week:

‘as elected politicians learn to exist in a much more open and critical society – the prime minister, chancellor and leader of the opposition have just published details of their tax affairs – the Queen and her immediate relatives are less open than ever to democratic scrutiny.’

The palace, under Elizabeth Windsor’s leadership, has demanded voluntary tax arrangements that must be kept secret. The royals have insisted on special treatment under the Freedom of Information Act, which was tightened up in 2010 to further limit access to information.

They have held onto the royal veto, which allows the Queen and Prince Charles to insist that laws affecting their private interests are tailored to suit their needs. That’s why so much legislation makes special provision for the Duchy of Cornwall, and why the Duchy continues to refuse to pay corporation tax.

You won’t find the Queen or Charles in the Panama Papers because their tax avoidance schemes are very much on shore and guarded by layers on acquiescent politicians and officials.

The palace also jealously guards its media image. This week’s interview with Prince William, where Nicholas Witchell asked about claims he was ‘work-shy’ was so obviously carefully planned, answers pre-prepared, they may as well have been sitting there reading directly from their scripts.

It was revealed just a few months ago that the royals insist on lengthy contracts being signed before agreeing to interviews, including the right to veto other contributors to programmes – a level of control no-one on the public payroll should ever have over their own scrutiny.

This week’s media coverage only reminds us that this is an institution that relies on make-believe and mythology to survive. The language of the discussion is all about ‘hard work’ and ‘duty’, we hear about charity, family and tradition. There is precious little serious challenge to the claims of the royals.

Instead we’re told how the Queen has managed to live so long, how William respects his grandmother, and we’re treated to photos of four generations of the family firm. This heavy handed PR is as much about the future as it is about the past – the royals know that a lot of their popularity is bound up with the Queen and that Charles presents a challenge to their future.

Prince Charles is a man on a mission. Joan Smith predicts he’ll be a ‘king in a hurry’, desperate to make his mark in the later years of his life. He is already widely acknowledged as an ‘activist’ prince, but he is a man who has no interest in the rules of democratic politics.

He demands he has his say but demands he isn’t challenged or scrutinized, as we’ve seen in recent years with his public interventions and his dogged determination to resist disclosure of his private lobbying.

So this 90^th birthday isn’t just a private matter, it matters to all of us. Because we are coming to the end of a reign of a monarch who has worked hard to preserve the interests of her own family and we are fast approaching the succession of a man determined to influence the agenda for this country.

Yet when that succession comes it won’t be through legitimate democratic means – it’ll be at the moment of another very personal event, the death of a monarch and the inheritance by her son of the highest public office in the country. That has to change.

Graham Smith is the chief executive officer of Republic