This blog follows the video analysis of Miller in which I promised to enlarge on the concept of royal prerogative (RP).
In the case (at para 47) the UKSC defined RP as ‘the residue of powers which remain vested in the Crown and they are exercisable by ministers, provided that the exercise is consistent with Parliamentary legislation’.
A key word here is ‘residue’. Put very simply there was a time when the Kingdom was governed by the monarch as a unitary authority (holding executive, legislative and judicial powers). Over time the monarch’s powers were eroded and other institutions such as Parliament and the Judiciary empowered (see e.g. Bill of Rights 1689). Also over time the day to day governing of the Kingdom passed from the monarch to the executive government (narrowly the Prime Minister and Cabinet). The modern executive takes its legal authority from two sources: statute (Acts of Parliament) and RP. The RP comprises those powers necessary for government, once held by the monarch, that have not been affected by statute. As RP powers are part of the common law they are inferior to statute and can be removed or altered by it.
Examples of RP powers wielded by the executive include declaring war and the making of and withdrawal from international treaties (this being the power examined in Miller). A number of powers have been retained by the monarch specifically, including the granting (or withholding) of royal assent to Acts of Parliament. (Although in reality, following a constitutional convention, the monarch will always follow the advice of ministers and accordingly assent will be given).
It was confirmed that prerogative powers are capable of review in CSSU v Min for Civil Service  (analysed here).
An issue for RP powers is that they are not always well defined (being creatures of history and the common law) and therefore one can ask the question of whether they are compatible with a modern parliamentary system based upon a rule of law in which the laws should be easily identifiable?