Christie’s will sell the richly illuminated tome in its valuable books and manuscripts sale on 8 July, with an estimate of £1.5 million to £2m. Dating from around 1290 to 1310, the work contains texts from the Old French Lancelot-Grail cycle and 126 miniature illustrations painted on vellum and decorated with gold leaf, including a rare depiction of the sorcerer Merlin transformed into a talking stag, and the Knights of the Round Table returning victorious from battle, The Guardian reports.

Known as the Lebaudy manuscript after the French industrialist Jean Lebaudy, who collected it in the 20th century, it has never been publicly exhibited or comprehensively studied.

Dr Eugenio Donadoni, Christie’s director of medieval and renaissance manuscripts, said only three such manuscripts are known to be in private collections, and theirs is the earliest, the most profusely illustrated and unique in its text.

The manuscript is attributed to the Master of the Liège Apocalypse, an anonymous but distinctive illuminator. Previous owners included a 15th-century jouster who died young and the bibliophile Sir Thomas Phillipps. Scholars hope it will finally enter a public collection, ending centuries of private ownership that has hampered research.