Common Brittonic (Welsh: Brythoneg; Cornish: Brythonek; Breton: Predeneg), also known as British, or Proto-Brittonic, is the reconstructed Celtic language thought to be historically spoken by the Celtic Britons in Britain and Brittany. It is the common ancestor of the later Brittonic languages.
It is a form of Insular Celtic, descended from Proto-Celtic, a theorized parent language that, by the first half of the first millennium BC, was diverging into separate dialects or languages. Evidence from early and modern Welsh shows that Common Brittonic was influenced by Latin during the Roman period, especially in terms related to the church and Christianity. By the sixth century AD, the languages of the Celtic Britons were swiftly diverging into Neo-Brittonic: Welsh, Cumbric, Cornish, Breton. Pictish may either have been a sister language or a descendant branch.