Brut y Brenhinedd

Adaptations of Geoffrey's Historia were extremely popular throughout Western Europe during the Middle Ages, but the Brut proved especially influential in medieval Wales, where it was largely regarded as an accurate account of the early history of the Celtic Britons.

Geoffrey's Historia Regum Britanniae (completed by c. 1139) purports to narrate the history of the Kings of Britain from its eponymous founder Brutus of Troy to Cadwaladr, the last in the line.

Geoffrey professed to have based his history on "a certain very ancient book" written in britannicus sermo (the "British tongue", i.e. Common Brittonic, Welsh, Cornish or Breton) which he had received from Walter of Oxford.

[12] This claim was taken up by the archaeologist Flinders Petrie, who argued in a paper presented to the Royal Society in 1917 that the Brut and the Historia Regum Britanniae were both derived from a hypothetical 10th-century version in Breton and ultimately from material originating in Roman times, and called for further study.

It survives in manuscripts dating from c. 1500, and Roberts argues that a "textual study of the version […] shows that this is a late compilation, not different in essentials from other chronicles which were being composed in the fifteenth century".