And the island of Britannia revolted from the Romans, and the soldiers there chose as their emperor Constantinus, a man of no mean station. And he straightway gathered a fleet of ships and a formidable army and invaded both Spain and Gaul with a great force, thinking to enslave these countries. . .the Romans never succeeded in recovering Britannia, but it remained from that time on under tyrants.
Source: Procopius, History of the Wars, 7 vols., trans. H. B. Dewing (Cambridge, Mass., and London: Harvard University Press & Wm. Heinemann, 1914; reprint ed., 1953-54), II.11-23.