Nicene and Post-Nicene Fathers, Ser. II, Vol I:
The Life of Constantine with Orations of Constantine and Eusebius.: Section 2

2. Constantine the Son of a British Princess.

Duke Coel of Colchester, say the old chronicles, by an insurrection became king. The Senate, rejoiced at the overthrow of an enemy, sent Constantius to Britain. Coel, fearing, sent ambassadors to meet him, gave hostages, and shortly died. Constantius was crowned, married Helena, daughter of Coel, the most beautiful, cultivated, and educated woman of her time. By her he had a son, Constantine, afterwards called the Great. This is in substance the account of Geoffrey of Monmouth (5. 6) and Pierre de Langloft (1, p. 66–7). The story is mentioned by p. 442 Henry of Huntington (Bk. I. 37), who perhaps wrote before Geoffrey (in 1137 [?]), and Richard of Cirencester (2. 1. 33). Waurin (Vol. I. Bk. 2. 43) makes “Choel” Count of Leicester, but in general is identical with Geoffrey. The famous Brut of Layamon (ed. Madden, 2 [1847] p. 35) is translated with amplifications from Wace’s Brut, and this in turn from Geoffrey. This makes Coel Earl of Gloucester. The Eulogium Hist. calls Helena (1. 337) daughter of a British king, but also concubine, though elsewhere (2, p. 267) she is wife according to the conventional story. It is also mentioned by many others; e.g. Voragine, Golden Legend. It is interesting that this legendary father of Helena is supposed (Hayden, Index to Eulogium, p. 45, and Giles, note on Geoffrey, p. 162) to be the same as “Old King Cole, the merry old soul,” making Constantine thus the grandson of the Mother Goose hero.