The national flag of the United Kingdom of Great Britain and Northern Ireland, usually called the Union Jack but more properly termed the Union Flag, gives symbolic expression to the union of England, Scotland and Northern Ireland under a single crown. Technically the Union Jack is a royal flag, and it has never been formally established in law as Britain's national flag. For many years its use as a national flag on land was merely customary. It was not until 1908 that it received official recognition in the form of a parliamentary statement that "the Union Jack should be regarded as the National flag." Later, in 1933, the Home Secretary, Sir John Gilmour, made a statement, generally accepted as authoritative, that "the Union Jack is the National Flag."
The Union Jack had its origins in a royal writ issued in 1606 by King James I. Already King James VI of Scotland, he ascended the English throne in 1603 upon the death of Elizabeth I, last of the Tudor monarchs. This union of the English and Scottish crowns was symbolized by the new flag described in the King's writ: 
By the King: Whereas, some differences hath arisen between Our subjects of South and North Britaine travelling by Seas, about the bearing of their Flagges: For the avoiding of all contentions hereafter. We have, with the advice of our Council, ordered: That from henceforth all our Subjects of this Isle and Kingdome of Great Britaine, and all our members thereof, shall beare in their main-toppe the Red Crosse, commonly called St. George’s Crosse, and the White Crosse, commonly called St. Andrew’s Crosse, joyned together according to the forme made by our heralds, and sent by Us to our Admerall to be published to our Subjects: and in their fore-toppe our Subjects of South Britaine shall weare the Red Crosse onely as they were wont, and our Subjects of North Britaine in their fore-toppe the White Crosse onely as they were accustomed.
The Union Jack was, therefore, conceived as a flag for use at sea. After the 1707 Act of Union that formally joined England and Scotland in the United Kingdom of Great Britain, it also began to be used on land, particularly as a component of the regimental colors of the British Army. In 1801, the Act of Union between Great Britain and Ireland led to a modification of the design. The red saltire cross on white, known as the Cross of St. Patrick, was added to symbolize the union of the "Three Kingdoms." In this form, the Union Jack has been used in many variations down to this day. The illustrations below show only a very small sample of the enormous number of flags and ensigns that are based on or incorporate the Union Jack.
See also History at the Hoist: Flags of the United Kingdom.


Some Variants of the Union Jack

Naval Ensign

Naval Reserve Ensign

Civil Ensign


Royal Fleet Auxiliary Ensign


Royal Air Force Ensign


Government Ensign


Lord-Lieutenant of a County


Guernsey Civil Ensign


Aberdeen Harbour Authority


Royal Victoria Yacht Club


House of Lords Yacht Club


Other National Flags Based on the Union Jack


New Zealand




Some Historical Flags Based on the Union Jack

British "Meteor Flag"
18th-Century Ensign

Taunton Flag

US Continental Colors

Canadian Red Ensign