In one illustration, painted on vellum and decorated with gold leaf, the sorcerer Merlin is depicted as a powerful shape-shifter who has transformed into a talking stag. In another, the Knights of the Round Table are shown returning, victorious, from battle.

The illustrations appear in one of the earliest manuscripts to tell the tale of King Arthur and the search for the holy grail – a richly illuminated medieval tome which, for more than 700 years, has been in private hands.

Now, institutions will have an opportunity to buy the work, which has never been publicly exhibited or comprehensively studied, when it is offered for sale at auction by Christie’s for an estimated £1.5m to £2m in July.

Dating from c1290 to 1310, the manuscript contains texts from the Old French Lancelot-Grail cycle – the foundational myths of King Arthur, Merlin and the holy grail – and 126 miniature illustrations, including a rare depiction of Merlin as a stag.

Only three such manuscripts are known to be held in private collections, said Dr Eugenio Donadoni, director of medieval and renaissance manuscripts at Christie’s, which is selling the work in its valuable books and manuscripts sale on 8 July: “Ours is the earliest of the three and the most profusely illustrated. Its text is unique.”

Known as the Lebaudy manuscript, after it was collected by the French industrialist Jean Lebaudy in the 20th century, it is thought to be the earliest copy of the Lancelot-Grail cycle ever to appear at auction.

“Many of the miniatures are in burnished gold, [created] using gold leaf, which was polished very aggressively to make it shine,” said Donadoni.

The manuscript is thought to have been produced at the end of the 13th century by the Master of the Liège Apocalypse, an anonymous but distinctive artist and illuminator named after the apocalyptic religious manuscript he was best known for. “He evidently was very sought after because the manuscripts he produced were very rich and would have cost a lot of money to make,” said Donadoni. “He’s got a very characteristic style and does these square-jawed male faces and lovely red dots on the cheeks of his characters.”

Previous owners of the manuscript have included a 15th-century knight, a jouster who died young, and the obsessive medievalist and bibliophile Sir Thomas Phillipps, while Lebaudy fought in two world wars and twice won the croix de guerre. “You can see how, throughout history, this manuscript might have appealed to these people,” Donadoni said.

Dr Irene Fabry-Tehranchi, who specialises in French texts at Cambridge University Library, said it was “amazing” to think that the Labaudy manuscript was being sold and could enter public ownership for the first time. “It’s really exciting. This private ownership of a key medieval manuscript is a real challenge for scholars. It prevents further research, unfortunately.”

Scholars have been unable to study the entire manuscript and its illustrations in depth, she said, because it has, perhaps understandably, been an object of desire for the super-rich. “It’s a beautifully illuminated manuscript, which contains wonderful Arthurian stories depicted in exquisite miniatures. Originally, it was made for aristocratic patrons, possibly women who were very fond of Arthurian stories.”

Now, she said, the manuscript is part of “our world heritage” and she is keeping her fingers crossed that, seven centuries later, the pattern of ownership can be broken. “I hope that … it will finally become available in a public collection. The problem is, of course, that nowadays libraries and heritage institutions don’t have very much money. [But] these institutions are there to preserve these texts and to make them publicly available.”

After examining scans of the manuscript, she said the ending of a classic story, known as the Suite Vulgate du Merlin, has been abridged and rewritten so that it presents a “special version” of the text.

“[At the end of] the standard version of the story, you have the development of knightly adventures and romance, and also a shift in focus from King Arthur to his nephew, the knight Gawain, and his adventures,” she said. “In the reworked version, I believe, of the Lebaudy manuscript – although we need to look at it more closely – we have more of a focus on the battles of Arthur and questions of kingship.”

This “different” ending better reflects the opening of the tale, where the king seeks to affirm his legitimacy while facing rebel British barons and fighting Saxon invaders.

“In some cases, scribes thought they could rewrite the story and make it more appealing to what either their patrons wanted or in a way that felt better [to them] – that was a standard medieval writing practice. It was probably someone’s initiative to do this rewriting, because they thought it was best,” she said. “It shows the scribe’s own take on the story.”