The Historia Britonum, or The History of the Britons, is a historical work that was first written sometime shortly after AD 820, and exists in several recensions of varying difference. (A recension is a critical revision of a text, i.e. different editors have worked upon the text to produce various versions.) It purports to relate the history of British inhabitants from earliest times, and this text has been used to write a history of both England and Wales, for want of more reliable sources.
The text itself is a collection of excerpts, chronological calculations, glosses, and summaries based on earlier records -- many of which no longer exist. As a result, the reliability of this work has been questioned both in part and in whole. The archeologist Leslie Alcock observed that in one recension of this manuscript the author called his work a heap of all he could find, and suggested that if we were to extend this metaphor, this text is "like a cairn of stones, uneven and ill-fitting... as an example of the historian's art it is atrocious. But it has the virtue of its defects. We can see the individual stones of the cairn, and in some cases we can trace the parent rock form which they came, and establish its age and soundness."
Traditionally, the Historia Britonum is ascribed to be the work of Nennius, a Welsh monk of the ninth century. However, examination of the numerous recensions show that Gildas was also claimed as its author (since Gildas was the only historical author its scribes knew of), while others (such as the British Library manuscript Harleian 3859) do not name an author. Professor Dumville's researches have shown that the ascription of this work to Nennius originated in the tenth century in one branch of the manuscript transmission, created by a scribe seeking to root this work in the intellectual traditions of that time.