The Britons of what is now Wales and the South-Western peninsula of Great Britain began to emigrate to Armorica. The history behind such an establishment is unclear, but medieval Breton, Angevin and Welsh sources connect it to a figure known as Conan Meriadoc. Welsh literary sources assert that Conan came to Armorica on the orders of the Roman usurper Magnus Maximus, who sent some of his British troops to Gaul to enforce his claims and settled them in Armorica. This account was supported by the Counts of Anjou, who claimed descent from a Roman soldier expelled from Lower Brittany by Conan on Magnus's orders.

Regardless of the truth of this story, Brythonic (British Celtic) settlement probably increased during the Anglo-Saxon invasion of Britain in the 5th and 6th centuries. Scholars such as Léon Fleuriot have suggested a two-wave model of migration from Britain which saw the emergence of an independent Breton people and established the dominance of the Brythonic Breton language in Armorica. Their petty kingdoms are now known by the names of the counties that succeeded them—Domnonée (Devon), Cornouaille (Cornwall), Léon (Caerleon); but these names in Breton and Latin are in most cases identical to their British homelands. (In Breton and French, however, Gwened or Vannetais continued the name of the indigenous Veneti.) Although the details remain confused, these colonies consisted of related and intermarried dynasties which repeatedly unified (as by the 7th-century Saint Judicaël) before splintering again according to Celtic inheritance practices.