The flag of England is the St George's Cross (heraldic blazon: Argent, a cross gules). The red cross appeared as an emblem of England in the Middle Ages. It also represents the official arms of the Most Noble Order of the Garter.
The flag was used as a naval ensign from at least the 16th century, and it was used as a component in the design of the Union Flag in 1606, but beyond this it has no official status in the United Kingdom. It began to be flown as the unofficial "national flag of England" from about the 1930s, and since the 1990s has been in increasingly wide use at sporting events.
In 1188 Henry II of England and Philip II of France agreed to go on a a crusade, and that Henry would use a white cross and Philip a red cross. 13th-century authorities are unanimous on the point that the English king adopted the white cross, and the French king the red one (and not vice versa as suggested by later use). It is not clear at what point the English exchanged the white cross for the red-on-white one.
There was a historiographical tradition claiming that Richard the Lionheart himself adopted both the flag and the patron saint from Genoa at some point during his crusade. This idea can be traced to the Victorian era, Perrin (1922) refers to it as a "common belief", and it is still popularly repeated today, even though it cannot be substantiated as historical.
Red crosses seem to have been used as a distinguishing mark worn by English soldiers from the reign of Edward I (1270s), or perhaps slightly earlier, in the Battle of Evesham of 1265, using a red cross on their uniforms to distinguish themselves from the white crosses used by the rebel barons at the Battle of Lewes a year earlier. Perrin (1922:37) notes a roll of accounts from 1277 where the purchase of cloth for the king's tailor is identified as destined for the manufacture of a large number of pennoncels (pennons attached to lances) and bracers (worn by archers on their left forearms) "of the arms of Saint George" for the use by the king's foot soldiers (pro peditibus regis). Perrin concludes from this that the introduction of the Cross of St George as a "national emblem" is originally due to Edward I. By 1300, there was also a greater "banner of St George", but not yet in a prominent function; the king used it among several banners of saints alongside the royal banner. Saint George had become popular as a "warrior saint" during the crusades, but the saint most closely associated with England was Edward the Confessor until the time of Edward III, who in thanks for Saint George's supposed intervention in his favour at the Battle of Crécy gave him a special position as a patron saint of the Order of the Garter in 1348. From that time, his banner was used with increasing prominence alongside the Royal Banner and became a fixed element in the hoist of the Royal Standard. The flag shown for England in the Book of All Kingdoms of 1367 is solid red (while St. George's Cross is shown for Nice and, in a five-cross version, for Tblisi). John Cabot, commissioned by Henry VII to sail "under our banners, flags and ensigns," reportedly took St. George's banner to Newfoundland in 1497.