The British Auxiliary Legion, also called the British Legion (La Legión Británica) 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.

Under the Quadruple Alliance, Great Britain had controlled maritime traffic along the Cantabrian coast since the beginning of the war.

In 1835 the war was not going well for the Liberal side and they asked their allies to become more involved in the war. The French sent their Foreign Legion which landed at Tarragona on 17 August with around 4,000 men and was renamed the Spanish Legion. Although the British refused to send troops directly, in June 1835, they decided to form a "military volunteer corps", and that became designated an auxiliary to the Spanish Legion. The corps was to be funded and their soldiers paid by the Spanish crown.

By the end of the summer of 1836 a force of 10,000 men under the command of De Lacy Evans had assembled in San Sebastian (Basque Country). They fought near Hernani and Vitoria, but were pushed back and had to hold the fort on Mount Urgull de San Sebastian, to prevent the Carlists from taking the city.

In November 1836 they were put under the command of Baldomero Espartero and helped to lift the siege of Bilbao.

In 1837 the Legion suffered a serious defeat in the Battle of Oriamendi, but later they led a successful offensive on the Carlist rearguard along the Biscay coast, launched in response to the Expedición Real, an attempt by the Carlists main force to march on Madrid. In the course of the offensive, Legion and Spanish loyalist forces managed to take over Irun.

The volunteers had signed on for a tour of two years and by July 1837 most of the men had returned home. The transport Prince Regent carried some 520 men to Greenock. The transport Parmelia carried 200 to Portsmouth.

Some 1,500–1,700 men remained under the name of New Legion, commanded by Brigadier-General Maurice O'Connell. Their casualties were so heavy, especially during their defeat at the battle of Andoain on 14 September, that in practical terms the new unit ceased to exist as an operational force after this action. Prince Regent carried some 600 men to Cork and Greenock. The New Legion was formally disbanded on 10 December 1837.^[1][2][3]

In total a quarter of the force – some 2,500 men – died, only half of them in combat.^[3] Their presence had not been well received by the Carlists; one former soldier wrote

"To our foes, we of the British Legion were the most odious of all; strangers, mercenaries, heretics, scoffers, polluters of their sacred soil; so they did term us. For us there was no quarter; in the heat of battle, or by cold judicial form, it was all the same: to fall into their hands was certainly a tortured death."^[4]

It would appear that most of the 9,600 men forming the Legion were volunteers from British Army line regiments. The only "regular" troops who served in the war were men of the Royal Navy, Royal Marines, Royal Artillery, and Royal Engineers. Recruiting and training began in 1835. The Legion was to comprise 8,500 infantry (in 10 battalions), 550 rifles, 700 cavalry and 300 artillery.

After the withdrawal of the British-commanded Auxiliary Legion and New Legion in 1837, a smaller 400-men unit of British volunteers was established in March 1838 within the ranks of the Spanish army. The battalion was led by Irish-born Spanish Colonel Federico Ricardo Lasaussaye and designed Brigada Auxiliar Británica ("British Auxiliary Brigade").^[10][11][12] Early into its existence, the Brigade was struck by a fever outbreak, which left one soldier dead and 30 hospitalised.^[13] The Brigade was made up of the remainders of Reina Isabel Lancers and the Legion's field battery.^[11][13] The unit, specially its cavalry component, was part of the Spanish forces that captured the Carlist stronghold of Barasoain, and successfully engaged the Carlist cavalry at Sesma.^[14]

• Foreign legion
• Foreign volunteers