It has generally been assumed that Celtic linguistic influence on Old English is limited to a few marginal loanwords. If a language shift had taken place from Celtic to Old English, however, one would expect to find traces of that in Old English phonology and (morpho)syntax. In this article I argue that (1) the way in which the West Germanic sound system was reshaped in Old English strongly suggests the operation of a hitherto unrecognized substratum; (2) that phonetic substratum is strongly reminiscent of Irish rather than British Celtic; (3) the Old Irish phonetic−phonological system provides a more plausible model for reconstructing the phonetics of pre-Roman Celtic in Britain than the British Celtic system. The conclusion is that there is phonetic continuity between pre-Roman British Celtic and Old English, which suggests the presence of a pre-Anglo-Saxon population shifting to Old English.