List of legendary kings of Britain in the context of "Matter of Britain"

Historia regum Britanniae (The History of the Kings of Britain), originally called De gestis Britonum (On the Deeds of the Britons), is a fictitious account of British history, written around 1136 by Geoffrey of Monmouth. It chronicles the lives of the kings of the Britons over the course of two thousand years, beginning with the Trojans founding the British nation and continuing until the Anglo-Saxons assumed control of much of Britain around the 7th century. It is one of the central pieces of the Matter of Britain.

Although taken as historical truth until the 16th century, it is now considered to have no value as history. When events described, such as Julius Caesar's invasions of Britain, can be corroborated from contemporary histories, Geoffrey's account can be seen to be wildly inaccurate. It remains, however, a valuable piece of medieval literature, which contains the earliest known version of the story of King Lear and his three daughters, and helped popularise the legend of King Arthur.

Latin versions of "King of Wales" (Welsh: Brenin Cymru) were titles used on a handful of occasions in the Middle Ages. They were very rarely claimed or applied by contemporaries, because Wales, much like Ireland, usually had neither the political unity nor the sovereignty of other contemporary European kingdoms such as England and Scotland. While many early rulers of areas within Wales used the title of "King", they were not, and did not claim to be, rulers of all Wales.

Geoffrey of Monmouth's History of the Kings of Britain achieved wide circulation from 1136. It has almost no historical value, but it popularised a fictitious list of legendary kings of Britain that remains central to the stories which make up the Matter of Britain.

Hafren (or Sabre, Sabren, or Sabrina; Welsh: Hafren; Old Welsh: Habren) is a legendary British princess who was drowned in the River Severn by her repudiated stepmother Gwendolen. The legend appears in Geoffrey of Monmouth's pseudohistorical chronicle Historia Regum Britanniae (c. 1136). According to Geoffrey, Hafren is the eponym of the Severn, which bears one of Britain's most ancient river names (recorded as early as the 2nd century in the Latinized form Sabrina).