Britannia Prima or Britannia I (Latin for "First Britain") was one of the provinces of the Diocese of "the Britains" created during the Diocletian Reforms at the end of the 3rd century. It was probably created after the defeat of the usurper Allectus by Constantius Chlorus in AD 296 and was mentioned in the c. 312 Verona List of the Roman provinces. Its position and capital remain uncertain, although it was probably located closer to Rome than Britannia II. At present, most scholars place Britannia I in Wales, Cornwall, and the lands connecting them. On the basis of a recovered inscription, its capital is now usually placed at Corinium of the Dobunni (Cirencester) but some emendations of the list of bishops attending the 315 Council of Arles would place a provincial capital in Isca (Caerleon) or Deva (Chester), which were known legionary bases.
Following the Roman conquest of Britain, it was administered as a single province from Camulodunum (Colchester) and then Londinium (London) until the Severan Reforms following the revolt of its governor Clodius Albinus. These divided the territory into Upper and Lower Britain (Britannia Superior and Inferior), whose respective capitals were at Londinium and Eboracum (York). During the first phases of the Diocletian Reforms, Britain was under the control of Allectus' Britannic Empire as part of the Carausian Revolt. At some point after the territory was retaken by Constantius Chlorus in AD 296, the Diocese of the Britains (with its vicar at Londinium) was established and made a part of the Prefecture of Gaul. The Britains were then divided among three, four, or five provinces, which seem to have borne the names Prima, Secunda, Maxima Caesariensis, and (possibly) Flavia Caesariensis and Valentia.
The placement and capitals of these late British provinces are uncertain, although the late-4th century List of Offices describes the governor of Prima as being equestrian rank (praeses), making the province unlikely to have been based in Londinium.
Describing the metropolitan sees of the early British church established by SS Fagan and "Duvian", Gerald of Wales placed Britannia Prima in Wales and western Britain, explaining its name by reference to the legendary Brutus's first settlements. Modern scholars disregard this gloss but generally agree in placing Britannia Prima in Wales, Cornwall (Cornubia), and the area connecting them. William Camden placed Prima to the south closest to Rome and this was generally accepted after the appearance of Charles Bertram's highly-influential 1740s forgery The Description of Britain, which gave Prima borders south of the Thames and the Bristol Channel; his work was, however, debunked over the course of the mid-19th century.
Owing to an inscription discovered at Corinium of the Dobunni (Cirencester) which refers to a rector of Britannia Prima named Lucius Septimius, Corinium is generally accounted as the provincial capital. The list of bishops who attended the 314 Council of Arles is patently corrupt but generally assumed to have mimicked the Roman administration: Camden proposed that Prima was based at London and Secunda at Caerleon and these were the two bishops apart from York. Bishop Stillingfleet and Thackery further proposed that scribal error had produced the bishop de colonia Londinensium ("from London colony") from original notes understood as Civ. Col. Londin. when Civ. Col. Leg. II (Caerleon) was intended. (Others place the bishop variously in Lincoln, Chester, and Colchester.) Another major settlement in the area was Glevum (Gloucester).
Ammianus records that in the year 369 Count Theodosius established or refounded the province of Valentia (further attested in the List of Offices) from lands recaptured from "the enemy". Its location is a matter of scholarly debate, but some place it in northwestern Wales with its capital at Deva (Chester). If so, it was probably intended to counter the extensive Irish piracy and raiding occurring in late antiquity.
- Polemius Silvius' 5th-century Nomina Omnium Provinciarum gives six provinces, but Roman administration over the Orcades (Orkneys) is generally discounted. Some modern scholars such as Birley, however, believe Maxima and Flavia were originally a single province Caesariensis which was later divided. This comports with Camden and some texts of Sextus Rufus, although they make the original province Britannia Maxima.
- Valentia is generally treated as a later formation and placed variously beyond the Wall, around the Wall, and in Wales. It may, however, have simply been another name for the British diocese as a whole.
- "Nomina Episcoporum, cum Clericis Suis, Quinam, et ex Quibus Provinciis, ad Arelatensem Synodum Convenerint" ["The Names of the Bishops with Their Clerics who Came Together at the Synod of Arles and from which Province They Came"] from the Consilia in Thackery (Latin)
- Frere, Sheppard (1967). Britannia: a history of Roman Britain. Cambridge: Harvard University Press. pp. 198–199.
- Camden, William (1610) [Original text published 1586], "The Division of Britaine", Britain, or, a Chorographicall Description of the most flourishing Kingdomes, England, Scotland, and Ireland, translated by Philemon Holland
- Dornier, Ann (1982). "The Province of Valentia". Britannia. 13: 253–260. doi:10.2307/526498.
- Notitia Dignitatum.
- Verona List.
- Giraldus Cambriensis [Gerald of Wales]. De Inuectionibus [On Invectives], Vol. II, Ch. I, in Y Cymmrodor: The Magazine of the Honourable Society of Cymmrodorion, Vol. XXX, pp. 130–1. George Simpson & Co. (Devizes), 1920. (Latin)
- Gerald of Wales. Translated by W.S. Davies as The Book of Invectives of Giraldus Cambrensis in Y Cymmrodor: The Magazine of the Honourable Society of Cymmrodorion, Vol. XXX, p. 16. George Simpson & Co. (Devizes), 1920.
- Creighton, John (2006). Britannia: the Creation of a Roman Province. London: Routledge.
- Hughes, William. The Geography of British History: A Geographical Description of the British Islands at Successive Periods from the Earliest Times to the Present Day: With a Sketch of the Commencement of Colonisation on the Part of the English Nation, p. 87. Longman, Green, Longman, Roberts, & Green (London), 1863.
- Labbé, Philippe & Gabriel Cossart (eds.) Sacrosancta Concilia ad Regiam Editionem Exacta: quae Nunc Quarta Parte Prodit Actior [The Sancrosanct Councils Exacted for the Royal Edition: which the Editors Now Produce in Four Parts], Vol. I: "Ab Initiis Æræ Christianæ ad Annum CCCXXIV" ["From the Beginning of the Christian Era to the Year 324"], col. 1429. The Typographical Society for Ecclesiastical Books (Paris), 1671.
- Thackery, Francis. Researches into the Ecclesiastical and Political State of Ancient Britain under the Roman Emperors: with Observations upon the Principal Events and Characters Connected with the Christian Religion, during the First Five Centuries, pp. 272 ff. T. Cadell (London), 1843.
- Stillingfleet, Edward. Origines Britannicæ: or, the Antiquities of the British Churches with a Preface, concerning Some Pretended Antiquities Relating to Britain, in Vindication of the Bishop of St. Asaph, New Ed., pp. 77 ff. Wm. Straker (London), 1840.
- Ammianus, XXVIII, iii.