Historia ecclesiastica gentis Anglorum
Historically, the predecessor to Constantine was Gratian on whom Geoffrey's tale was probably based. The Venerable Bede refers to this Gratian as Municeps in his "Historia ecclesiastica gentis Anglorum" (Latin: "Ecclesiastic History of the English People") in Chapter XI of this work and the epithet is seemingly there to distinguish this Gratian from the earlier Gratian killed by the Usurper Magnus Maximus.