Declarations of war by Great Britain and the United Kingdom

A declaration of war is a formal declaration issued by a national government indicating that a state of war exists between that nation and another. In the United Kingdom, only the monarch has the power to declare war and peace, under the royal prerogative. There have been no declarations of war since the Second World War, though British Armed Forces have taken part in armed conflict on numerous occasions nonetheless.

There has been a long-running debate regarding whether Parliament alone should have the power to declare war and more widely to commit British forces to armed conflict. This was attempted (to the limited extent of possible war against Iraq) in 1999 with the introduction of the Military Action Against Iraq (Parliamentary Approval) Bill. However Queen Elizabeth II, acting upon the advice of her government at the time, refused to grant her consent to allow the bill to be debated in Parliament and so it was dropped (Queen's Consent was needed before debate could take place because the bill affected the royal prerogative). The Constitutional Reform and Governance Act 2010 originally included a section that would have required Parliamentary approval for use of the armed forces, but this was dropped from the bill before royal assent.

Formal declarations of war by the Kingdom of Great Britain
The following table refers to declarations of war from the Act of Union in 1707 until the creation of the United Kingdom of Great Britain and Ireland in 1801.

Formal declarations of war by the United Kingdom
The following table refers to declarations of war since the creation of the United Kingdom in 1801. In 1927 the United Kingdom of Great Britain and Ireland was renamed the "United Kingdom of Great Britain and Northern Ireland".