Clearly, that Jeremy Corbyn will stop at nothing. This time he's picking on the dear old Queen. Just as the nation prepares to celebrate the monarch's record-breaking reigning, Labour's Robespierre-in-waiting rolls out the tumbrel.

Well, up to a point, citoyen. Though some headlines have suggested that Mr Corbyn has attacked “the Queen's powers”, he has been a little more subtle. For one thing, unaccountably, he has not called for the monarchy's abolition. He has not attempted to disturb the royal personage, or force her family to make alternative lifestyle choices. The Crown, though, is another matter.

Many people, like many headline writers, are confused by the distinction. When the Queen marks 63 years, seven months and two days on the throne – a week today, if you're ordering bunting – she will have outlasted Victoria in the tenacious monarch stakes. She will also be celebrating the survival of a most peculiar system. That, though he's hardly the first, is Mr Corbyn's beef.

In this odd arrangement, the Queen has powers and no powers simultaneously. She can do nothing herself, except “in theory”, but some things, quite a number of things, cannot be done without her. She has no personal capacity in government, but a lot of government only happens when she is involved in person.

Then there is the difference between the symbolic and the actual, the monarch and the Crown, powers vested and powers exercised “under advice” from ministers. There is a device called the Royal prerogative involving all sorts of authority and immunities, and another device called orders in council, meaning the Privy Council. Mr Corbyn isn't happy about either. Just to help matters along, no one has quite managed to define the prerogative. Whether orders in council can be challenged in court is another grey area.

Clear? Of course not. The lack of clarity has nothing to do with a lack of brainpower among generations of constitutional lawyers. Crudely, governments do what monarchs used to do while pretending that the monarch is still giving orders or, at least, giving orders under advice she cannot reject. (Unless, of course, she can, in certain circumstances.)

There are plenty of other nuances. There is a history of learned dispute, too, over what it once meant to be a monarch and how that affects the prerogative today. Some believe all this adds up to the most wonderful set of constitutional arrangements ever cobbled together. Mr Corbyn, it is safe to say, is not of that school. The Queen, while “assenting” to laws and talking of “my government” decade after decade, has offered no opinion. This, at least, is wise.

Mr Corbyn's point is simple. After 63 years, governments are still getting away with ignoring parliament by using the prerogative and orders in council. This becomes important when ministers are being sacked or appointed, treaties negotiated, or wars started. Orders in council can be – have been – a means of making laws by decree. Mr Corbyn, and numerous others, thinks this makes a nonsense of parliament.

As he said at a hustings last week: “The Royal prerogative should be subject to parliamentary vote and veto if necessary. The Queen hands her powers to the Prime Minister and he can then exercise them. It’s a very convenient way of by-passing parliament. Also, orders in council are a very convenient way of by-passing parliament.”

“Very convenient” is certainly true. The extent of the convenience becomes clear when you realise that, supposedly, the monarch and the Privy Council (“the Queen-in-Council”) exercise the prerogative and issue those orders. In reality, the Cabinet is a “committee” of the council, makes all policy decisions, and does the ordering. Parliament, in those circumstances, is rendered irrelevant.

For those wedded to a constitutional monarchy, this is awkward. If the Queen alone was using all the old powers of royalty we would not pretend that democracy existed in Britain. But when a cabinet gives autocracy a run for its money, a fiction is treated to a nod and a wink. That Cabinet members have been elected is no excuse: they have their jobs because they were chosen as members of a parliament. The occasional presence of peers in their ranks only makes matters worse.

But still: abuses are mostly minor, are they not? And hasn't this convenient system always worked well enough, most of the time? That would be a matter of opinion. Even if it is your opinion, and even if you are not too picky over what you choose to call democracy, Mr Corbyn's demand for a veto is hard to dispute. Isn't a parliament supposed to exist for precisely this kind of reason?

There has been a predictably anguished cry from monarchists. If Labour's man from the Committee of Public Safety got his way, what job would be left for the Queen? Sir Anthony Seldon, historian, headmaster of Wellington, and – sheer coincidence – a biographer of Tony Blair, said this week: “It would be crossing the Rubicon. He [Mr Corbyn] would be the first Labour leader who started talking about a reduction in the role of the monarchy. It would be very serious.”

No doubt. But it might also come as a revelation to many, after 63 years, that the monarchy is not merely ceremonial, or decorative, or a useful device in a modern democracy. If so little power attaches to the Queen, and if parliament matters, why be alarmed by the idea that MPs could have a vote on laws made behind the scenes?

By next week there will be plenty of voices to tell you that these have been 63 glorious years. The Queen, we can all agree, has had what is generally called a good innings. If you like, you will be able to commemorate her staying power with the purchase of a special £20 silver coin. At less than three hours on the new national living wage, that's surely a bargain, and something to remember her by if Sir Anthony's worst fears are realised.

You cannot fault his logic, after all. If Mr Corbyn has a decent argument on his side, and if there is a chance that the Islington MP could yet become Prime Minister, why would a role remain for the monarchy? And why shouldn't the Queen's eventual passing mark the end of a constitutional era, if not the end of the monarchy itself? The prerogative is a bizarre survival from a pre-democratic age.

Someone should break the news to the ageing Prince Charles. Letters ending “I have the honour to be, Sir, Your Royal Highness’s most humble and obedient servant” are a kind of comic footnote to Britain's fixation with royalty. Even Alex Salmond, the author in question, realised eventually that the salutation was “inappropriate to a democratic age”. You could say the same, many times over, of the monarchical apparatus that hangs around the British state like a row of gaudy medals.

For students of the constitution, arguments over the prerogative are old hat. For the rest of us, they reappear with every manufactured royal celebration, reminders that behind the pomp and sentiment democracy is manipulated in the same old ways. And there is nothing sentimental about it.