The design of the Union flag is defined by the Proclamation Declaring His Majesty's Pleasure concerning the Royal Style and Titles appertaining to the Imperial Crown of the United Kingdom of Great Britain and Ireland, and its Dependencies, and also the Ensigns, Armorial Flags, and Banners thereof, which was dated 1 January 1801.

This states that the Union flag shall be Azure, the Crosses Saltires of St. Andrew and St. Patrick quarterly per saltire, counterchanged argent and gules; the latter fimbriated of the second, surmounted by the Cross of St. George of the third, fimbriated as the Saltire.

There are no fixed colours in heraldry: provided the blue and red used in the flag are clearly identifiable as such, any shade chosen is acceptable. The proclamation makes no statement as to the proportions of the flag or of its constituent parts; many different forms of the flag are correct.

For the avoidance of doubt and the sake of convenience, Garter King of Arms, under the authority of the Earl Marshal, has approved two versions of the Union flag as being accurate representations suitable for use. These are of the proportions 5:3, commonly flown on land; and 2:1, commonly flown at sea.

Union flag with proportions 5:3

Union flag with proportions 2:1