A hand-sewn, white linen Spanish flag, with six eyelets in the hoist. It is painted with the Spanish Royal Arms in the centre — Castile (a yellow castle on a red field) and Leon (a white field with a red, crowned, rampant lion), the Bourbon arms — blue bearing three gold fleur-de-lis in escutcheon; a crown above and collar of the Order of the Golden Fleece surrounding the cartouche, the Burgundian saltire overall.

The flag is believed to have been captured during the siege of Gibraltar in 1782 and presented to Lieutenant James Robert Mosse (1746-1801). Mosse was killed at the battle of Copenhagen on 2 April 1801 while in command of HMS 'Monarch' and is commemorated by a public monument in St Paul's Cathedral.