Prior to about the fifth century CE, most people in what is now England spoke Celtic languages (for the most part specifically Brittonic languages), although Vulgar Latin may have taken over in larger settlements, especially in the south-east. The fundamental reason for the death of these languages in early medieval England was the arrival in Britain of immigrants who spoke the Germanic language now known as Old English, particularly around the fifth century CE. Gradually, Celtic-speakers switched to speaking this English language until Celtic languages were no longer extensively spoken in England.
However, the precise processes by which this shift happened have been much debated, not least because the situation was strikingly different from, for example, post-Roman Gaul, Iberia, or North Africa, where Germanic-speaking invaders gradually switched to local languages. Explaining the rise of Old English is therefore crucial in any account of cultural change in post-Roman Britain, and in particular to understanding the Anglo-Saxon settlement of Britain, and is an important aspect of the history of English as well as the history of the Celtic languages.
The modern scholarly consensus is that the prime reason for the death of Brittonic in most of England was that in the fifth to sixth centuries, the new political elite of Old English-speaking immigrants made it politically expedient for other people in Britain to adopt their new rulers' language and Anglo-Saxon ethnicity.
Fairly extensive information about language in Roman Britain is available from Roman administrative documents attesting to place- and personal-names, along with archaeological finds such as coins, the Bloomberg and Vindolanda tablets, and Bath curse tablets. This shows that most inhabitants spoke British Celtic and/or British Latin until the Roman economy and administrative structures collapsed, around the early fifth century.
However, by the eighth century, when extensive evidence for the language situation in England is next available, it is clear that the dominant language was Old English, whose West Germanic ancestors were spoken in what is now the Netherlands, north-western Germany, and southern Denmark. There is no serious doubt that Old English was brought to Britain primarily during the fifth and sixth centuries by migrants from those regions.
Because the main evidence for events in Britain during the crucial period 400–700 is archaeological, and seldom reveals linguistic information, while 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):
Although Cumbric, in the north-west, seems to have died during the eleventh century, Cornish continued to thrive until the early modern period, retreating at only around 10 km per century. But from about 1500, Cornish-English bilingualism became increasingly common, and Cornish retreated at more like 30 km per century. Cornish fell out of use entirely during the eighteenth century, though the last few decades have seen an attempted revival.