A term of constitutional law in jurisdictions who have retained the vestiges of the English monarchy, referring to the oft-political and always discretionary decisions which must and do vest in the head of state or her/his executive committee often referred to as privy council or even cabinet.
In Operation Dismantle Inc. v. R., Justice Marceau of the Federal Court of Appeal of Canada wrote, at ¶71:
"It is well known that, historically, the royal prerogative is what has been left to the King from the wide discretionary powers he enjoyed at the time he governed as an absolute monarch, powers which the great statutes of the constitutional history of England (among which those expressly referred to by the learned Judge in his decision, the Bill of Rights, 1688 and the Act of Settlement, 1700) were aiming at defining and containing by proceeding to solemn declarations of the fundamental laws of England.
"The idea that certain privileges, freedoms and powers remained directly associated with the dignity and responsibility of the Crown persisted even after the royal authority had become totally subject to the supremacy of Parliament, except that these royal prerogatives were then seen as arising out of the common law and their content, not defined a priori, became subject to the will of the elected representatives of the people, free to intervene at any time to clarify their content or limit their extent. A host of statutes were adopted over the years to regulate areas which had until then been matters of royal discretion, but certain privileges and powers - including most of those connected with the supreme command of armed forces, national defence and the conduct of external affairs (including declarations of war) - have always been left intact. It is these privileges and powers which today make up the royal prerogative, which privileges and powers have continued to be exercised apart from and independently of Parliament - in the sense that, although they continue to exist by the will of Parliament, they do not derive from it - and which, once it was established that the King would act only on the advice of his Ministers, became in reality the privileges and powers of the government, and so of the Cabinet....
"There is no doubt that the royal prerogative so defined and characterized exists in Canada in the same way as in England ... just as there is no doubt that, in principle, neither in Canada nor in the United Kingdom can the royal prerogative be limited except by clear and express statutory provision."
The royal prerogative is the last remaining bastion of the full gamut of statutory authority and discretion once vested in a monarch, slowly whittled won by centuries of parliamentary and democratic reform (and the actions of more than a few bad monarchs such as Charles II, Henry VIII and Louis XVI), to a bare necessity required to enable a government to make and implement necessary political decisions and to fashion an administration around it to ensure the prompt and reliable delivery and administrative support of the elected government's political platform or ideology.
On an ongoing basis, the royal prerogative cedes to statutes which continually erodes it, such as, for example, statutes which prohibit deficit budgets or which set the term of a government to four years (as opposed to allowing the prorogation of a parliamentary assembly at the call of a premier or prime minister and a subsequent opportune election).
In Ross River Dena Council Band v Canada, Justice LeBel adopted these words at ¶54:
"Generally speaking, in my view, the royal prerogative means the powers and privileges accorded by the common law to the Crown.
"The royal prerogative is confined to executive governmental powers, whether federal or provincial.
"The extent of its authority can be abolished or limited by statute. Once a statute has occupied the ground formerly occupied by the prerogative, the Crown has to comply with the terms of the statute”.
In Turp, Justice Noël of the Federal Court wrote:
"Under the royal prerogative, the conduct of foreign affairs and international relations, including the decision to conclude or withdraw from a treaty, falls exclusively under the executive branch of government. n the absence of a Charter challenge, it appears that a decision made in the exercise of prerogative powers would not be justiciable.... the royal prerogative power could be abolished or limited by a legislative provision."
In Attorney-General v. De Keyser’s Royal Hotel Ltd., the judgment of the House of Lords included these words:
"Inasmuch as the Crown is a party to every Act of Parliament it is logical enough to consider that when the Act deals with something which before the Act could be effected by the prerogative, and specially empowers the Crown to do the same thing, but subject to conditions, the Crown assents to that, and by that Act, to the prerogative being curtailed....
“The Royal Prerogative has of necessity been gradually curtailed, as a settled rule of law has taken the place of an uncertain and arbitrary administrative discretion”.
"In summary, then, as statute law expands and encroaches upon the purview of the royal prerogative, to that extent the royal prerogative contracts. However, this displacement occurs only to the extent that the statute does so explicitly or by necessary implication."
Examples abound of the royal prerogative and include the management of war and of armies, content and tabling of government bills in the House of Commons, the appointment of heads of departments such as deputy ministers, the appointments of individuals to sit as judges in courts of law and the membership of administrative tribunals and Crown corporations, of the chairs or members of parliamentary committees, of the appointment of senators, the members of a privy council (cabinet) and even some which are controversial still, such as the selection of lawyers to the designation of queen's counsel.
- Attorney General (on behalf of His Majesty) v De Keyser’s Royal Hotel Ltd,  AC 580
- Operation Dismantle Inc. v. R., 3 DLR (4th) 193 (1983)
- Ross River Dena Council Band v Canada, 2002 SCC 54
- Turp v Minister of Justice and Attorney General of Canada, 2012 FC 893