In the dedication to his Historia Regum Britanniae, Geoffrey claims that while writing the book he had struggled to find material on the early Kings of the Britons.
This problem had been solved when Walter gave him a "very ancient book" written in britannicus sermo (The "British" tongue, i.e. Brittonic, Welsh, or Breton).
"[4] Some antiquarians, notably Flinders Petrie in 1917, suggested that Walter's original Welsh source for the Brut Tysilio was in fact the "ancient book" described by Geoffrey.
[5] However, modern scholarship has established that all variants of the Brut y Brenhinedd in fact originate with Geoffrey's work, and do not represent some prior chronicle tradition.
[6] Indeed, the Brut Tysilio probably postdates Walter by centuries; Brynley F. Roberts argues that it is an "amalgam" of previous versions and came together around 1500.