The Monarchist League is the name of a number of groups in Commonwealth Realms that are dedicated to the preservation of their country's existing constitutional monarchy system of government.
All Commonwealth Realms operate under the titular rule of the British Monarch, who is given a different style in each country (Queen of Canada, Queen of New Zealand, etc). Commonwealth Monarchist Leagues promote the British Monarch as an independent and respectable Head of State who can effectively represent their nation, while at the same time be "above" partisan politics. They may also try to promote and engage in the study of the concept of monarchy, the roles of the Crown in their country's system of democratic government, and the formative influence of the Royal Family on society.
Commonwealth Monarchist Leagues are also often staunch advocates of the preservation of British customs, traditions, and symbols in government. For example, the Australian Monarchist League has been a vocal opponent of the movement to remove the Union Jack from the Flag of Australia.
