The Council of Civil Service Unions claimed in judicial review that the order defeated their legitimate expectation of being able to collectively bargain for fair wages.

The House of Lords held that exercises of the royal prerogative were subject to judicial review, but there were exceptions, including for matters of national security.

After a spy scandal in 1983, the organisation became known to the public, and Margaret Thatcher's government decided a year later that employees would not be allowed to join a trade union for reasons of national security.

Despite an extensive publicity campaign by trade unions, the government refused to reverse its decision but instead offered affected employees the choice between £1,000 and the membership of a staff association or dismissal.

[10] Lord Diplock said the following: My Lords, I intend no discourtesy to counsel when I say that, intellectual interest apart, in answering the question of law raised in this appeal, I have derived little practical assistance from learned and esoteric analyses of the precise legal nature, boundaries and historical origin of "the prerogative," or of what powers exercisable by executive officers acting on behalf of central government that are not shared by private citizens qualify for inclusion under this particular label.

It does not, for instance, seem to me to matter whether today the right of the executive government that happens to be in power to dismiss without notice any member of the home civil service upon which perforce it must rely for the administration of its policies, and the correlative disability of the executive government that is in power to agree with a civil servant that his service should be on terms that did not make him subject to instant dismissal, should be ascribed to "the prerogative" or merely to a consequence of the survival, for entirely different reasons, of a rule of constitutional law whose origin is to be found in the theory that those by whom the administration of the realm is carried on do so as personal servants of the monarch who can dismiss them at will, because the King can do no wrong.

Nevertheless, whatever label may be attached to them there have unquestionably survived into the present day a residue of miscellaneous fields of law in which the executive government retains decision-making powers that are not dependent upon any statutory authority but nevertheless have consequences on the private rights or legitimate expectations of other persons which would render the decision subject to judicial review if the power of the decision-maker to make them were statutory in origin.

From matters so relatively minor as the grant of pardons to condemned criminals, of honours to the good and great, of corporate personality to deserving bodies of persons, and of bounty from moneys made available to the executive government by Parliament, they extend to matters so vital to the survival and welfare of the nation as the conduct of relations with foreign states and – what lies at the heart of the present case – the defence of the realm against potential enemies.

I have in mind particularly the possible adoption in the future of the principle of "proportionality" which is recognised in the administrative law of several of our fellow members of the European Economic Community; but to dispose of the instant case the three already well-established heads that I have mentioned will suffice.

Whether he has or not is par excellence a justiciable question to be decided, in the event of dispute, by those persons, the judges, by whom the judicial power of the state is exercisable.

The reasons for the decision-maker taking one course rather than another do not normally involve questions to which, if disputed, the judicial process is adapted to provide the right answer, by which I mean that the kind of evidence that is admissible under judicial procedures and the way in which it has to be adduced tend to exclude from the attention of the court competing policy considerations which, if the executive discretion is to be wisely exercised, need to be weighed against one another – a balancing exercise which judges by their upbringing and experience are ill-qualified to perform.

[12] It is, I hope, not out of place in this connection to quote a letter written in 1896 by the great legal historian F. W. Maitland to Dicey himself: "the only direct utility of legal history (I say nothing of its thrilling interest) lies in the lesson that each generation has an enormous power of shaping its own law": see Richard A. Cosgrove, The Rule of Law; Albert Venn Dicey; Victorian Jurist (1980), p.177.

In my view the exercise of the prerogative which enabled the oral instructions of 22 December 1983 to be given does not by reason of its subject matter fall within what for want of a better phrase I would call the "excluded categories" some of which I have just mentioned.

It follows that in principle I can see no reason why those instructions should not be the subject of judicial review.In the exertion therefore of those prerogatives, which the law has given him, the King is irresistible and absolute, according to the forms of the constitution.

[14] The GCHQ case, therefore, was highly important since it held that the application of judicial review would be dependent on the nature of the government's powers, not their source.

In R (Bancoult) v Secretary of State for Foreign and Commonwealth Affairs (No 2), heard at the House of Lords, one of the matters decided was whether or not the courts could subject Orders in Council to judicial review.

As such, the Lords saw "no reason why prerogative legislation should not be subject to review on ordinary principles of legality, rationality and procedural impropriety in the same way as any other executive action".

[20] In R (Miller) v The Prime Minister and Cherry v Advocate General for Scotland [2019] UKSC 41 the Supreme Court quashed an Order in Council that sought to prorogue Parliament.