Queen of the Commonwealth

The Queen of the Commonwealth was the title given to the head of the Commonwealth. (PROSE: Head Games)

Elizabeth II, who primarily reigned as the Queen of the United Kingdom, (PROSE: The Dying Days) was a Queen of the Commonwealth. (PROSE: Head Games) She acceded to the throne following the death of her father, King George VI, on 6 February 1952. (PROSE: A History of Humankind) Barring a brief period in 1997 during which she was deposed by the Ice Warrior Xznaal, (PROSE: The Dying Days) Elizabeth ruled continuously until the early part of the 21st century, (PROSE: The Shadows of Avalon, Mad Dogs and Englishmen, The Edge of Glory, AUDIO: The Longest Night, A Death in the Family, Black and White) after which she was succeeded by Charles III. (PROSE: Revenge of the Judoon)

Elizabeth was also the Queen of Australia, (AUDIO: The Children of Seth) formally named the Commonwealth of Australia, (TV: Children of Earth: Day Three) a situation with which not all Australians were satisfied. Tegan Jovanka favoured her country's transition from a constitutional monarchy to a republic. (AUDIO: The Children of Seth)

The Eleventh Doctor told the Prince of Wales that his mother, the Queen, was busy with matters of the Commonwealth, whilst she had infact been transformed into a goldfish. (HOMEVID: Bad Night)

Behind the scenes
In the real world, the title "Queen of the Commonwealth" does not exist, with the head of the organisation simply known as the "Head of the Commonwealth". However, of the fifty-four member states, sixteen (including Australia) have Elizabeth II as their head of state while thirty-three are republics and the remaining five have different monarchs.

Although no other heads of the Commonwealth have been confirmed to exist within the Doctor Who universe, George VI was the first holder of the postion and the to-be Charles III has been designated as Elizabeth's heir. This matches both the real world and in-universe chronology of the monarchs of the United Kingdom.