A copy of William Shakespeare's First Folio, the first compilation of the Bard's plays published in 1623, has been discovered in the library of an ancient port town in northern France.

One of the world's most valuable and coveted books, the First Folio was uncovered when librarian Remy Cordonnier dusted off a copy of Shakespeare's works thought to be from the 18th century for an exhibition on English literature in the town of Saint-Omer near Calais.

"It occurred to me that it could be an unidentified First Folio, with historic importance and great intellectual value," he said.