Who should declare war

It's a fact of our constitution that under the Royal Prerogative the power to declare war and commit British forces to military operations is vested in the prime minister. Parliament has no formal legal role in sanctioning such use although the government, by convention, does undertake to keep Parliament substantially informed. It may be posited that in exercising this particular prerogative the prime minister enjoys what ancient kings once enjoyed: the use of almost absolute power in the application of a discretion, and that in doing so he knows that such use cannot generally be challenged in the courts or stopped by a Parliament controlled by his party.

With some prerogatives, control by judicial review is possible, however, the courts have limited their use of it on the basis that some matters involving the use of the prerogative are issues of “high policy”, including declaring and conducting war, and are not justiciable.

Lord Roskill in Council of Civil Service