The British Auxiliary Legion, also called the British Legion or Westminster Legion, existed from 1835 to 1837. It was a British military force sent to Spain to support the Liberals and Queen Isabella II of Spain against the Carlists in the First Carlist War.

The First Carlist War was a civil war in Spain from 1833 to 1840, the first of three Carlist Wars. It was fought between two factions over the succession to the throne and the nature of the Spanish monarchy: the conservative and devolutionist supporters of the late king's brother, Carlos de Borbón, became known as Carlists (carlistas), while the progressive and centralist supporters of the regent, Maria Christina, acting for Isabella II of Spain, were called Liberals (liberales), cristinos or isabelinos. Aside from being a war of succession about the question who the rightful successor to king Ferdinand VII of Spain was, the Carlists’ goal was the return to a traditional monarchy, while the Liberals sought to defend the constitutional monarchy.