The earliest reference to the cross of St George as an English emblem (not as a flag) was in an account relating to the Welsh War of 1277. The flag shows the Cross of Saint George, who was the patron saint of England and Wales. During King Edward III's reign (1327-1377) Saint George was made patron of Edward's new knight-order, the Order of the Garter. From the 14th century onwards St George was considered a special protector of the English. All this led to the cross of Saint George being taken as a symbol during the crusades, however there are several theories about exactly how this came to be.History
The first theory is that the flag was adopted during the Crusades. At the beginning of the Crusades, a red cross on white was already associated with England. Although the Pope decided English crusaders would be distinguished by wearing a white cross on red, and French crusaders a red cross on white English knights soon decided to claim "their" cross of red on white. In January 1188, in a meeting between Henry II of England and Philip II of France, the two rivals agreed to exchange flags (France later changed its new white cross on red for a white cross on a dark blue flag). Some French knights carried on using the red cross however, and as English knights wore this pattern as well, the red cross on white became the typical crusader symbol regardless of nationality.
A second theory states that St. George's cross was originally the flag of Genoa and was adopted by England and the City of London in 1190. They did this so their ships entering the Mediterranean could benefit from the protection of the powerful Genoese fleet. The maritime Republic of Genoa was rising and going to become, with its rival Venice, one of the most important powers in the world. The English Monarch paid an annual tribute to the Doge of Genoa for this privilege.
St George's cross did not achieve any status as the national flag until the 16th century, when all other saints' banners were abandoned during the Reformation under Edward VI.
The Flag of England is part of the Union Flag. The Union Flag has been used in a variety of forms since 1606, when the flags of the Kingdom of Scotland and Kingdom of England were first merged during the Union of the Crowns. In Scotland historical evidence suggests that a separate design of the Union Flag was flown to that used in England. Following the Acts of Union of 1707, which properly united the Kingdom of Scotland and the Kingdom of England, the "English" version of the Union Flag was adopted as the offical flag of England, Wales and Scotland.
From 1801, in order to symbolise the union of the Kingdom of Great Britain with the Kingdom of Ireland, a new design which included St Patrick's Cross was adopted for the flag of the United Kingdom. This design is what we know today as the flag of the United Kingdom.