Britonia is the historical name of a settlement in Galicia (northwestern Spain) which was settled in the late 5th and early 6th centuries AD by Romano-Britons escaping the advancing Anglo-Saxons who were conquering Britain at the time. Britonia is therefore similar to Brittany in Gaul in that it was settled by expatriate Britons at roughly the same time.
What little is known of Britonia is deduced from its religious history. The British settlements were recognised at the Council of Lugo in 567 and a separate bishopric established. Mailoc was nominated Bishop of Britonia and signed the acta at the Second Council of Braga in 572. The British Celtic settlements were quickly integrated and their adherence to Celtic rite lasted only until the Fourth Council of Toledo in 633 decreed the now so-called Visigothic or Mozarabic rite as the standard liturgy of Hispania. The see of Britonia existed at least until 830 when the area was attacked by the Vikings; it may have continued as late as the Council of Oviedo in 900. It was finally merged with the Diocese of Mondoñedo-Ferrol.
Known bishops of the ecclesia Brittaniensis include: