Geoffrey of Monmouth in his History of the Kings of Britain, attempted to list all of the kings of Britain reigning between the arrival of Brutus and the Britons (a possible, archaeology-supported, early influx of Continental Celts) in c.1100 BC up to AD 689 and the end of Gwynedd's attempts to regain the land lost to the Anglo-Saxons. Nominal rulers of the British Celtic tribes (starting initially in the south and east of Britain and working northwards), they were probably rulers of their own tribal groups and held at least theoretical over-kingship over the rest. They usually only exercised real authority in this role in times of emergency, such as at the landings of Julius Caesar in 55 and 54 BC.

Pre-Roman, heroic age Celtic kingdoms almost always formed the basis of the ancestral heritage of later Post-Roman Celtic kings. Although largely legendary until elements of the Roman and Post-Roman periods, considering the importance that the Celtic warrior class placed on lineage, added to the tradition of being able to recite one's ancestors, these names may well reflect an element of truth. But, because the Pre-Roman Celts left no written records, and the Post-Roman Britons probably lost a large amount of whatever records they possessed after the Adventus Saxonum, very little of this can ever be proved. The dates shown here are rough approximations for the legendary period (lilac-backed), and are calculated back from known High Kings. They should not be taken as being historically accurate.

Note: Some later High Kings, accepted as such in other references, are not on Geoffrey's list, and are shown here in maroon text. The listing of most of these British monarchs was derived by the late Lewis Thorpe, PhD from the 1966 translation of The History of the Kings of Britain (1982 edition). Additional text kindly supplied by Mick Baker.

c.1100 BC

- 156

383 - 388

?388 - ?406

406

406 - 411

c.480 - 511

634 - 664