Prophetiae Merlini

The Prophetiae preceded Geoffrey's larger Historia Regum Britanniæ of c. 1136, and was mostly incorporated in it, in Book VII;[4] the prophecies, however, were influential and widely circulated in their own right.

[6] When Geoffrey's Historia was largely translated by Wace into the Roman de Brut, he omitted the material on Merlin's prophecies, though he does profess knowledge of them.

[6] Many of its prophecies referring to historical and political events up to Geoffrey's lifetime can be identified – for example, the sinking of the White Ship in 1120, when William Adelin, son of Henry I, died.

[13] Alternatively this may preserve the Breton or Cornish original to which he may have been referring; John of Cornwall's (1141–55) version is notable for its localisation in the southwestern region known to Gildas and Nennius as Dumnonia.

The first work about the prophet Myrddin in a language other than Welsh, the Prophetiae was widely read — and believed — much as the prophecies of Nostradamus were centuries later; John Jay Parry and Robert Caldwell note that the Prophetiae Merlini "were taken most seriously, even by the learned and worldly wise, in many nations", and list examples of this credulity as late as 1445.

[15] At much the same time, and in the same area, Abbot Suger copied some of the prophecies almost exactly in his Life of Louis the Fat, for the purpose of praising Henry I of England.

On the other hand, they had their defenders, and there was a revival of Arthurian lore with a Protestant slant, used in particular by John Dee to develop the concept of the British Empire in the New World.

By the 17th century Geoffrey's history in general, and Merlin's prophecies in particular, had become largely discredited as fabrications, for example as attacked by William Perkins.