Flavia Caesariensis (Latin for "The Caesarian province of Flavius"), sometimes known as Britannia Flavia, 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. It seems to have been named after Chlorus's family and was probably located beside Maxima Caesariensis, but their positions and capitals remain uncertain. At present, most scholars place Flavia Caesariensis in the southern Pennines, possibly reaching the Irish Sea and including the lands of the Iceni. Its capital is sometimes placed at Lindum Colonia (Lincoln).
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 the Allectus's 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 formed and made a part of Prefecture of Gaul. The Britains were 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 Notitia Dignitatum lists the governor (praeses) of Flavia being of equestrian rank, making it unlikely to have been based in Londinium. The list of bishops who attended the 314 Council of Arles is patently corrupt but generally assumed to have mimicked the Roman administration: the identification of Lindum Colonia as a provincial capital rests on proposed emendations of one or the other of the bishops from the cities Londinensi and colonia Londinensium. Those emendations are highly speculative: Bishop Ussher proposed Colonia, Selden Camaloden or Camalodon, and Spelman Camalodunum (all various names of Colchester); Camden took it as Caerleon, with Bishop Stillingfleet and Thackery proposing that a scribal error created Civ. Col. Londin. from an original Civ. Col. Leg. II (Caerleon).
Describing the metropolitan sees of the early British church established by SS Fagan and "Duvian", Gerald of Wales placed Flavia around London, extending into Mercia. Bertram's highly-influential forgery The Description of Britain placed it similarly: although not including London, it included central England and was bound by the Severn, the Thames, the North Sea, and the Humber and Mersey; this was accepted for a century from the 1740s to the 1840s before being revealed as a forgery. Modern scholars usually place Londinium in Maxima rather than Flavia. Birley has argued that Maxima and Flavia originally consisted of a single province, which received the name Britannia Caesariensis as a mark of favour for support against the rebel Allectus in 296. Although Flavia is usually thought to have been formed from the old province of Lower Britain, Birley proposes that Upper Britain was divided in two (between Prima and Caesariensis) and then three (Prima, Maxima, and Flavia). This repeats Camden's earlier theory (relying on Sextus Rufus) that Maxima was formed first and Flavia followed sometime after. Supporters of a later creation of Flavia note that it need not refer to Constantius Chlorus himself: instead, it may have honored any of Constantine, Valentinian, or Theodosius.
- 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
- Polemius Silvius's 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.
- Dornier, Ann (1982). "The Province of Valentia". Britannia. 13: 253–260. doi:10.2307/526498.
- Notitia Dignitatum.
- Verona List.
- "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)
- 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.
- 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.
- 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.