Prior to the 5th century AD, most people in Great Britain spoke a Brittonic language, but the number of these speakers declined sharply throughout the Anglo-Saxon period (between the 5th and 11th centuries), when Brittonic languages were displaced by Old English.

Debate continues over whether this change was due to mass migration or to a small-scale military takeover by people who spoke the West Germanic dialects that became Old English. The situation in post-Roman Britain was certainly strikingly different from, for example, post-Roman Gaul, Iberia or North Africa, where invaders speaking other languages gradually switched to local languages.Bryan Ward-Perkins, ‘Why did the Anglo-Saxons not become more British?’, English Historical Review, 115 (2000), 513–33. (p. 517)Chris Wickham, Framing the Early Middle Ages: Europe and the Mediterranean 400–800 (Oxford: Oxford University Press, 2005), pp. 311–12.Hills, C. M. (2013). "Anglo-Saxon Migrations". The Encyclopedia of Global Human Migration. Wiley-Blackwell. . This linguistic decline is therefore crucial to understanding the cultural changes in post-Roman Britain, the Anglo-Saxon settlement of Britain and the emergence of an English language.

The notable exceptions were the Cornish language persisting into the 18th century, and a form of Welsh remaining in common usage in the English counties along the Welsh border into the late 19th century.Transactions Woolhope Naturalists' Field Club, 1887, page 173 (reprinted as );

Chronology

Fairly extensive information about language in Roman Britain is available from Roman administrative documents attesting to personal and place-names, along with archaeological finds such as coins, the Bloomberg and Vindolanda tablets, and Bath curse tablets. That shows that most inhabitants spoke British Celtic and/or British Latin. The influence and position of British Latin declined when the Roman economy and administrative structures collapsed in the early 5th century.Kenneth Hurlstone Jackson, Language and History in Early Britain: A Chronological Survey of the Brittonic Languages, First to Twelfth Century A.D., Edinburgh University Publications, Language and Literature, 4 (Edinburgh: Edinburgh University Press, 1953).A. L. F. Rivet and Colin Smith, The Place-Names of Roman Britain (London: Batsford, 1979).Paul Russell, 'Latin and British in Roman and Post-Roman Britain: methodology and morphology', Transactions of the Royal Philological Society, 109.2 (July 2011), 138–57.

There is little direct evidence for the linguistic situation in Britain for the next few centuries. However, by the 8th century, when extensive evidence for the language situation in England is next available, it is clear that the dominant language was what is today known as Old English. There is no serious doubt that Old English was brought to Britain primarily during the 5th and 6th centuries by settlers from what are now the Netherlands, north-western Germany, and southern Denmark who spoke various dialects of Germanic languages and who came to be known as Anglo-Saxons. The language that emerged from the dialects they brought to Britain is today known as Old English. There is evidence for Britons moving westward, and also across the English Channel to Brittany, but those who remained in what became England switched to speaking Old English until Celtic languages were no longer extensively spoken there. Celtic languages continued to be spoken in other parts of the British Isles, such as Wales, Scotland, Ireland and Cornwall. Only a few English words of Brittonic origin appear to have entered Old English.

Because the main evidence for events in Britain during the crucial period (400–700) is archaeological and seldom reveals linguistic information, and written evidence even after 700 remains patchy, the precise chronology of the spread of Old English is uncertain. However, Kenneth Jackson combined historical information from texts like Bede's Ecclesiastical History of the English People (731) with evidence for the linguistic origins of British river names to suggest the following chronology, which remains broadly accepted (see map):

In Area 1, Celtic names are rare and confined to large and medium-sized rivers. This area corresponds to English language dominance up to c. 500–550. Area 2 shows English-language dominance c. 600. Area 3, where even many small streams have Brittonic names, shows English-language dominance c. 700. In Area 4, Brittonic remained the dominant language until at least the Norman Conquest, and river names are overwhelmingly Celtic.Kenneth Hurlstone Jackson, Language and History in Early Britain: A Chronological Survey of the Brittonic Languages, First to Twelfth Century A.D., Edinburgh University Publications, Language and Literature, 4 (Edinburgh: Edinburgh University Press, 1953), p. 220.

Although Cumbric, in the northern part of Area 3, seems to have died during the 11th century,Diana Whaley, A Dictionary of Lake District Place-names, Regional series (English Place-Name Society), 1 (Nottingham: English Place-Name Society, 2006), esp. pp. xix–xxi. Cornish continued to thrive until the early modern period and retreated at only around 10 km per century. However, from about 1500, Cornish–English bilingualism became increasingly common, and Cornish retreated at closer to 30 km per century. Cornish fell out of use entirely during the 18th century, though the last few decades have seen an attempted revival.Ken George, 'Cornish', in The Celtic Languages, ed. by Martin J. Ball and James Fife (London: Routledge, 1993), pp. 410–68 (pp. 411–15). Welsh continued to be spoken in some western parts of Herefordshire and Shropshire into modern times.

During that period, England was also home to influential communities speaking Latin, Old Irish, Old Norse and Anglo-Norman. None of those seem to have been a major long-term competitor to English and Brittonic, however.

Debate on whether British Celtic was being displaced by Latin before the arrival of English

As of around 2010 there was an ongoing discussion about the character of British Celtic and the extent of Latin-speaking in Roman Britain.David N. Parsons, 'Sabrina in the thorns: place-names as evidence for British and Latin in Roman Britain', Transactions of the Royal Philological Society, 109.2 (July 2011), 113–37.Peter Schrijver, Language Contact and the Origins of the Germanic Languages, Routledge Studies in Linguistics, 13 (New York: Routledge, 2014), pp. 31–91. Scholars agreed that British Latin was spoken as a native language in Roman Britain and that at least some of the dramatic changes that the Brittonic languages underwent around the 6th century were due to Latin speakers switching language to Celtic,D. Gary Miller, External Influences on English: From Its Beginnings to the Renaissance (Oxford: Oxford University Press, 2012), pp. 25–28. possibly as Latin speakers moved away from encroaching Germanic-speaking settlers.Peter Schrijver, Language Contact and the Origins of the Germanic Languages, Routledge Studies in Linguistics, 13 (New York: Routledge, 2014), pp. 31–48. It was thought likely that Latin was the language of most of the townspeople; the administration and the ruling class; the military and the church. Some scholars thought that British Celtic probably remained the language of the peasantry, which was the bulk of the population; the rural elite was probably bilingual. But others suggested that Latin became the prevalent language of lowland Britain, in which case the story of Celtic language death in what is now England begins with its extensive displacement by Latin.Peter Schrijver, ‘The Rise and Fall of British Latin: Evidence from English and Brittonic’, in The Celtic Roots of English, ed. by Markku Filppula, Juhani Klemola and Heli Pitkänen, Studies in Languages, 37 (Joensuu: University of Joensuu, Faculty of Humanities, 2002), pp. 87–110.Peter Schrijver, ‘What Britons spoke around 400 AD’, in N. J. Higham (ed.), Britons in Anglo-Saxon England (Woodbridge: Boydell, 2007), pp. 165–71.

Thomas Toon has suggested that if the population of Roman Lowland Britain was bilingual in both Brittonic and Latin, such a multilingual society might adapt to the use of a third language, such as that spoken by the Germanic Anglo-Saxons, more readily than would a monoglot population.Toon, T.E. The Politics of Early Old English Sound Change, 1983.

Debate on the reason for the Brittonic language's minuscule influence on English

Old English shows little obvious influence from Celtic: there are vanishingly few English words of Brittonic origin.Kastovsky, Dieter, ‘Semantics and Vocabulary’, in The Cambridge History of the English Language, Volume 1: The Beginnings to 1066, ed. by Richard M. Hogg (Cambridge: Cambridge University Press, 1992), pp. 290–408 (pp. 301–20).Matthew Townend, 'Contacts and Conflicts: Latin, Norse, and French', in The Oxford History of English, ed. by Lynda Mugglestone, rev. edn (Oxford: Oxford University Press, 2012), pp. 75–105 (pp. 78–80).A. Wollmann, 'Lateinisch-Altenglische Lehnbeziehungen im 5. und 6. Jahrhundert', in Britain 400–600, ed. by A. Bammesberger and A. Wollmann, Anglistische Forschungen, 205 (Heidelberg: Winter, 1990), pp. 373–96. Latin loanwords into early Old English were more numerous; since they were part of a continuous process of borrowing from Latin into Germanic languages, it is hard to be sure how many belong to the early Old English period, but they number in the tens or hundreds.Alfred Wollmann, 'Lateinisch-Altenglische Lehnbeziehungen im 5. und 6. Jahrhundert', in Britain 400–600, ed. by Alfred Bammesberger and Alfred Wollmann, Anglistische Forschungen, 205 (Heidelberg: Winter, 1990), pp. 373–96.Alfred Wollman, 'Early Latin Loan-words in Old English', Anglo-Saxon England, 49 (1993), 1–26.Anna Helene Feulner, Die griechischen Lehnwörter im Altenglishen, Münchener Universitäts-Schriften, 21 (Frankfurt am Main: Lang, 2000), .

Demographic Replacement Theory

The traditional explanation for the lack of Celtic influence on English, supported by uncritical readings of the accounts of Gildas and Bede, is that Old English became dominant primarily because Germanic-speaking invaders killed, chased away, and/or enslaved the previous inhabitants of the areas that they settled, particularly in the East. A number of specialists maintained support for such explanations into the 21st century,D. Hooke, 'The Anglo-Saxons in England in the seventh and eighth centuries: aspects of location in space', in The Anglo-Saxons from the Migration Period to the Eighth Century: an Ethnographic Perspective, ed. by J. Hines (Woodbridge: Boydell, 1997), 64–99 (p. 68).O. J. Padel. 2007. “Place-names and the Saxon conquest of Devon and Cornwall.” In Britons in Anglo-Saxon England [Publications of the Manchester Centre for Anglo-Saxon Studies 7], N. Higham (ed.), 215–230. Woodbridge: Boydell. and variations on this theme continued to feature in standard histories of English.Ferdinand von Mengden, “Old English,” in English Historical Linguistics, Vol.1 (ed. Alexander Bergs, Laurel J. Brinton, 2012), p. 22Seth Lerer, Inventing English (Columbia University Press: 2007), p. 9Richard Hogg, Rhona Alcorn, An Introduction to Old English, 2012, pp. 3–4Haruko Momma, Michael Matto, A Companion to the History of the English Language, 2011, p. 154 Peter Schrijver said in 2014 that "to a large extent, it is linguistics that is responsible for thinking in terms of drastic scenarios" about demographic change in late Roman Britain.Peter Schrijver, Language Contact and the Origins of the Germanic Languages, Routledge Studies in Linguistics, 13 (New York: Routledge, 2014), quoting p. 16.

Elite acculturation