The Battle of Trafalgar remains one of the most spectacular and consequential naval victories in history, ending the threat of an invasion of Britain by Napoleon, and setting the scene for a century or more of British dominance of the seas. It was achieved in the face of the numerical superiority of the allied French and Spanish fleets, which had six more ships of the line, carrying 420 more guns and almost double the manpower of the British fleet: the victory was due to the unorthodox tactics of Admiral Horatio Nelson, directing his fleet in two columns to cut the extended Franco-Spanish line at right angles, thus inducing a mêlée in which the superior seamanship, gunnery and morale of the British fleet were decisive. Twenty of the 33 ships in the Franco-Spanish fleet were captured: the British fleet lost none. Nelson's death during the battle is still one of the emblematic moments of British history.

This Union Flag would have been flown prominently by HMS Spartiate, most likely from the foretopgallant stay, as she sailed slowly towards the waiting line of the Franco-Spanish fleet off Cape Trafalgar. Under ordinary circumstances Union Jacks were flown only when in port, from a jack staff on the ship's bowsprit, and their use in battle at Trafalgar was in accordance with a specific order issued by Nelson only 11 days beforehand, on 10 October 1805: 'When in presence of the Enemy, all the ships under my command are to bear White Colours [i.e. the White Ensign] and a Union Jack is to be suspended from the foretopgallant stay'. This was to enable the British ships to recognise each other in the confusion of battle: Nelson insisted on a yellow and black paint scheme for his ships, the 'Nelson Chequer', for the same reason. The Spartiate may possibly have flown more than one Union Jack: a midshipman on the British HMS Neptune, ahead of Spartiate in the windward column, remembered looking back and seeing 'union-jacks and ensigns ... made fast to the fore and fore-topmast-stays, as well as to the mizzen-rigging, besides one at the peak, in order that we might not mistake each other in the smoke' (Roy Adkins. Trafalgar: The Biography of a Battle, 2004, 97).

The 74-gun HMS Spartiate was originally a French ship, and formed part of the fleet defeated by Nelson at the Battle of the Nile on 1 August 1798: supposedly it was from Spartiate that Nelson received the wound to his forehead at the battle. Captured at the Nile, she was commissioned into the British Navy in the following year. At Trafalgar, she was commanded by Captain (later Admiral) Sir Francis Laforey, 2nd Bart, under whom she had taken part in the unsuccessful pursuit of the French fleet across the Atlantic in 1804 and the subsequent blockade of Cadiz. Along with HMS Minotaur, the Spartiate was at the rear of Nelson's northern, windward column at Trafalgar, and the slow rate of sailing of both ships was to prevent them from joining the action until some two hours after Collingwood aboard Royal Sovereign had been the first to breach the Franco-Spanish line. However, shortly after 2.30pm they played a significant role in driving off the four unengaged ships of the French vanguard under Rear-Admiral Dumanoir Le Pelley, firing broadsides into Dumanoir's Formidable. At around 4.30pm, the two ships then engaged the Spanish 84-gun Neptuno, which surrendered after a fight of around an hour. The Spartiate sustained three killed and 20 wounded: according to The Trafalgar Roll she 'had her foretopsail yard shot away, and her masts, yards, and rigging in general were a good deal damaged'. After Trafalgar Spartiate served at Rochefort in 1807-08, and in June 1809 took part in the capture of the islands of Ischia and Procida off Naples: she continued in active service until 1835, before being made a sheer hulk at Plymouth in 1842, where she was broken up in 1857.

It was common practice for warrant officers or junior officers to take ensigns as rewards after a battle: the Spartiate's second lieutenant, James Clephan (1768-1851) preserved the present flag. Born in Fife, Clephan had been apprenticed in the merchant service before being pressganged into the navy in 1794 as an able seaman: he was made Master's Mate in 1795, and still held that rank aboard the Doris in 1801 when he distinguished himself in the cutting out of the Chevrette near Brest, whereupon he was promoted lieutenant for 'his distinguished gallantry on that memorable occasion, when, although knocked overboard while ascending her side, he was the first to gain the enemy’s deck, and there received several slight wounds' (W.R. O'Byrne. A Naval Biographical Dictionary): Admiral Cornwallis is reported to have commented of his promotion that 'few officers have earned it so hardly'. Clephan became first lieutenant of Spartiate immediately after Trafalgar, at the special request of Captain Laforey. He was made Commander in 1811, and in the War of 1812 against the United States whilst commanding the sloop Charybdis captured the American privateer Blockade near the Isle of Saba in the Caribbean; he also served in the unsuccessful assault on New Orleans in 1814-15. He went on half-pay on 23 August 1815 and retired with the rank of Captain in 1840, one of only a handful of men to have reached that rank from the lower decks during the Napoleonic period.

Only two other complete British flags are known to survive from Trafalgar, both Union Jacks: the first, from HMS Minotaur (which sailed alongside Spartiate into action) was preserved by Stephen Hilton, Master's Mate, and subsequently presented by his descendants to the church of St Mary’s, Selling, Kent, from whom it was purchased privately by the National Maritime Museum in 2012, together with an Austrian Ensign thought to have been retrieved by Hilton from the Spanish Neptuno. The second, from HMS Royal Sovereign, was kept by Charles Antram, also Master's Mate, and remained in private hands until publicly identified in May 2025. Minotaur's flag is of comparable size to Spartiate's at 226.5 x 311cm, having suffered a loss to its right-hand edge, and with an oblong section cut away from the lower edge, perhaps as a souvenir; the Union Jack from Royal Sovereign is smaller, at 152.5 x 274.5cm. Fragments of various sizes survive from one of the Union Jacks of HMS Victory, which was cut up for souvenirs by the honour guard of sailors after Nelson's funeral: a substantial fragment measuring 88 x 94 cm sold at Sotheby's, 17 January 2018, lot 94 (£297,000).

The Union Flags flown at Trafalgar are the second (and still current) form of the flag, introduced on 1 January 1801 after the 1800 Act of Union with Ireland: the earlier design lacked the cross of St Patrick (the red saltire), as can be seen in the surviving flag flown by HMS Queen Charlotte at the Glorious First of June, 1794 (sold at Christie's, 8 November 2006, lot 24). As a relatively recent innovation, the red saltire is often incorrectly executed in examples from the early 19th century, reflecting the handmade nature of flags at this period: here the white and red saltires are not counterchanged as they should be. The flag shows signs of battle damage: analysis by the Zaricor Flag Collection revealed shards of metal embedded in the fabric in several places, notably in the half-moon shaped loss to one edge, suggesting that this is the 'footprint' of a cannon ball. Fragments of wood splinters were also found throughout.