The UK High Court has ruled that the process for the UK to leave the EU, including when, must be made by Parliament and not the government. The government was pursuing the exit from the EU pursuant to what is called the “Royal Prerogative”. The High Court ruled that the government does not have power under the Royal Prerogative to override legislation and exercise the withdrawal from the EU.
The Royal Prerogative is law dating back to the medieval monarchy. In a nutshell, it preserves in the Crown, residual, discretionary and arbitrary powers that can be exercised in numerous circumstances. In regular language, Royal Prerogative means the power that the King or Queen of England retained when Absolute Reign was abolished in favor of a democracy in the UK. In respect of Canada, because we inherited the laws of the UK, we also inherited the Royal Prerogative and it remains in force in Canada because we have the UK monarchy as part of our constitutional legal regime.
The Royal Prerogative powers cannot be catalogued but key areas include powers relating to the legislature, the judicial system, foreign affairs, defence of the realm, conferring state honors and appointments, state immunity, emergency powers and parens patriae role. Parens Patriae is an area of law that gives the Crown power over people who are legally incompetent (such as children who are too young to understand legal consequences).
The Royal Prerogative includes both domestic and international jurisdiction. In Canada, for example, the issuance of a passport is a domestic exercise of jurisdiction given pursuant to the Royal Prerogative. Canada could legislate in respect of the right to issue passports but it has more power to keep that area as a Royal Prerogative.