thumb|right|300px|[[Roman Britain around AD 410, without speculative provincial borders.]]
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 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.
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left|200px|thumb|The traditional arrangement of the late Roman provinces after [[William Camden|Camden, placing Prima along England's southern coast. On the basis of modern archaeology, the province at least reached as far north as Corinium.]]
left|200px|thumb|Another possible arrangement of the late Roman provinces, with Prima in Wales and Cornwall
History
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,
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 ('), 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. William Camden placed Prima to the south closest to Rome his work was, however, debunked over the course of the mid-19th century.
thumb|Inscription of Lucius Septimius
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