The following list of legendary kings of Britain derives predominantly from Geoffrey of Monmouth's circa 1136 work Historia Regum Britanniae ("the History of the Kings of Britain"). Geoffrey constructed a largely fictional history for the Britons (ancestors of the Welsh, the Cornish and the Bretons), partly based on the work of earlier medieval historians like Gildas, Nennius and Bede, partly from Welsh genealogies and saints' lives, partly from sources now lost and unidentifiable, and partly from his own imagination. A parallel work, which may predate the Historia, is known as the Brut y Bryttaniait. The Brut differs from the Historia in several key areas and does not contain any of Geoffrey's characteristic embellishments. However, they both, in the large part draw from the same now lost original source material. Several of his kings are based on genuine historical figures, but appear in unhistorical narratives. A number of Middle Welsh versions of Geoffrey's Historia exist. All post-date Geoffrey's text, but may give us some insight into any native traditions Geoffrey may have drawn on.
Geoffrey's narrative begins with the exiled Trojan prince Brutus, after whom Britain is supposedly named, a tradition previously recorded in less elaborate form in the 9th century Historia Brittonum. Brutus is a descendant of Aeneas, the legendary Trojan founder of Rome, and his story is evidently related to Roman foundation legends.
The kings before Brutus come from a document purporting to trace the travels of Noah in Europe and once attributed to the Mesopotamian historian Berossus, but now known to have been a fabrication of the 15th century Italian writer Annio da Viterbo. Renaissance historians like John Bale and Raphael Holinshed took the list of kings of Celtica given by pseudo-Berossus and made them kings of Britain as well as Gaul. John Milton records these traditions in his History of Britain, although he gives them little credence.
Geoffrey synchronises some of his kings with figures and events from the Bible, Greek, Roman and Irish legends, and recorded history. These are given in the "Synchronisation" column.
After the death of Cadwallader, the kings of the Brythons were reduced to such a small domain that they ceased to be kings of the whole Brythonic-speaking area. Two of his relatives, Yvor and Yni, led the exiles back from Brittany, but were unable to re-establish a united kingship. The Anglo-Saxon invaders ruled the south-eastern part of the island of Great Britain, which would become England, after that point in time under the Bretwaldas and later the kings of England.
The heirs to the Celtic-British throne continued through the Welsh kings of Gwynedd until that line was forced to submit itself to the English in the 13th century. Princes and lords of Gwynedd ruled until the reign of Dafydd III, who ruled from 1282 to 1283. His death marked the end of the house of Brutus. Owen Tudor, grandfather of Henry VII of England, was a maternal descendant of the kings of Gwynedd; Henry's marriage with Elizabeth of York thus signified the merging of the two royal houses (as well as the feuding houses of York and Lancaster).