William V

William V was the king of the United Kingdom who succeeded Charles III. When the Tenth Doctor found that Edward VII had disappeared along with Balmoral Castle in 1902, he warned that the future of the royal family, along with the rest of history, was threatened, saying that, along with the rest of his successors, there would be no William V. (PROSE: Revenge of the Judoon)

During the Martian Invasion on 6 May 1997, which fell within the reign of Queen Elizabeth II, the Prince of Wales and his sons were on the royal jet on course to London when the Martian ship entered the atmosphere, diverting to Madrid. (PROSE: The Dying Days)

A member of the Silence was present at his wedding to his wife during the reign of Elizabeth II. (PROSE: The Secret Lives of Monsters)

Searching through the pocket of the Ninth Doctor's leather jacket, Rose Tyler found a £10 coin which claimed to show the head of William V. (PROSE: Winner Takes All)

Behind the scenes

 * William is possibly the unnamed King of the year 2050 mentioned in the K9 television story Oroborus; however, it is also possible that that king is his father, Charles III. (Assuming that Charles III is the actual, he would be 102 in 2050; this may seem unlikely, but did live to the age of 101.)
 * When an heir accedes to the British throne, they may choose the name they will rule under, and it does not necessarily have to be their birth name. There is no guarantee that when William takes the throne in real life that he will, in fact, choose to be William V.