King of the Britons

The title King of the Britons (Brenin y Brythoniaid, Rex Britannorum) was used (often retrospectively) to refer to a ruler, especially one who might be regarded as the most powerful, among the Celtic Britons, both before and after the period of Roman Britain up until the Norman invasion of Wales and the Norman conquest of England. Britons were the Brittonic-speaking (ancestral language of Welsh) peoples of what is now Wales, England and southern Scotland. The Britons contributed as ethnic ancestors of the native British population including the Welsh, Cornish, English and Scottish people but also of the Bretons.

During the Norman and Plantagenet periods, only Wales (or parts thereof) remained under Brittonic rule in Britain and the term "Britons" (Brythoniaid, Britaniaid, Brutaniaid) was used in Britain to mean the Welsh people (Cymry in modern Welsh). This, and the diminishing power of the Welsh rulers relative to the Kings of England, is reflected in the gradual evolution of the titles by which these rulers were known from "King of the Britons" in the 11th century to "Prince of Wales" in the 13th.