Few commentators expected that the UK Supreme Court would deliver a unanimous and unequivocal reply to the arguments which Gina Miller, Joanna Cherry and others brought against the decision by Boris Johnson, the prime minister, to advise the Queen to prorogue parliament. For constitutional lawyers, it was indeed exciting to see Brenda Hale, the president of the court, lay down the building blocks upon which a decision of great significance was established. One-word descriptions of the UK constitution as being essentially “l(fā)egal” or “political” do not bring out the significance of history in our national life. And if our system of democratic government is the result of history, then it is clear that the development has never ceased.

The doctrine of the royal prerogative has sometimes been revered through its link with the monarch. But as long ago as 1611, the English court held that the sovereign “hath no prerogative but that which the law of the land allows him”. In 1820, Joseph Chitty wrote a volume entitled A Treatise on the Law of the Prerogatives of the Crown; and the relative duties and rights of the subject. Chitty explained that the power to prorogue parliament was vested in the king “by the constitution, that is, by the common law, and by the custom of parliament”. Two hundred years later, we now know that the common law and the custom of parliament are still potent sources of our constitution.

The Supreme Court had to deal with the following issues. First, was the issue of the lawfulness of the prime minister’s advice to the Queen justiciable in the courts? Second, if it was so justiciable, by what standard should it be judged? Third, applying that standard, was the advice lawful? And finally, if the advice was not lawful, what remedy should be granted?