Celtic language decline in England

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.

Source: Wikipedia — Celtic language decline in England (CC BY-SA 4.0)