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.

Although the United Kingdom (Great Britain from 1707 until 1801) has been involved in a great number of wars throughout the years, they have only formally declared war 23 times.

The UK or Great Britain has declared war on the following countries: France (4), Spain (4), Germany (2), Bulgaria (2), Netherlands (1), Russia (1), Zulu Nation (1), Austria-Hungary (1), Ottoman Empire (1), Italy (1), Finland (1), Hungary (1), Romania (1), Japan (1), and Thailand (1).

Although the United Kingdom went to war against the United States, there was actually no official declaration of war.

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