Saturday marks the anniversary of the Queen's accession in 1952. I hate to say it, but as a constitutional monarch, she has been pathetic. Over her reign, she has allowed government politicians to accumulate frightening power. She has merely stood by as they cast aside all restraint, including the basic rights, liberties and institutions that were fought for precisely to protect us from arbitrary authority.

At first, of course, they were intended to protect us from the power of absolute monarchs. In time, though, Parliament replaced the monarch as sovereign; but these same rules worked equally well at restraining politicians too. Ministers knew that they were only the temporary custodians of the public trust; and that their power was checked and balanced by MPs, the civil service, and the courts.

Indeed, the monarchy itself became one of these balancing institutions. It may seem bizarre in a democracy that the monarch is notionally the head of the government, the church, the peerage and the army; but the reason we keep it that way is not so that monarchs can wield power, but so as to keep unlimited power out of the hands of politicians. For most of the time, our monarchs have had a better grasp of the mood of the people, and of the importance of their rights and freedoms, than have ministers: so this has proved a useful arrangement.

The key constitutional role of monarchs today, then, is to stop politicians from usurping power and turning themselves into an elected dictatorship. But the Queen – perhaps confusing the exercise of this role with political interference – has allowed precisely that to happen. With Magna Carta, the Queen’s distant ancestor agreed to fundamental principles such as our right not to be held without trial, and to be tried by a jury. Yet in her own reign (starting perhaps in 1971 with internment in Northern Ireland, but escalating fast in the last dozen years) these rights, and more, have simply been signed away.

The constitutional role of an unelected, hereditary monarchy must be limited. But it does have a constitutional role, and must exercise that role as a necessary counterweight to the otherwise unbridled power of an executive that – through its majority and its patronage – is in complete control of Parliament. It is time for both Palace and Parliament to initiate a genuine public debate on that role, and on when and how the monarchy should legitimately intervene to ensure that the rights and civil liberties of the people are preserved.

Reprinted from Dr Butler's new book The Alternative Manifesto (Gibson Square Books).