Spanish police are investigating the disappearance of the Codex Calixtinus, a valuable 12th century manuscript, from the Santiago de Compostela cathedral in northwestern region of Galicia, a spokesman said on Thursday.

The manuscript is a collection of sermons and liturgical texts and served as a guide for the historical Camino de Santiago pilgrimage, which dates back to the Middle Ages.

The elaborately illustrated document disappeared from a safe deposit box in the cathedral last week.

Its suspected theft, only reported to police on Wednesday, is considered a major loss for Spain''s cultural and religious heritage.

Santiago Cathedral is the reputed burial place of Saint James the Greater, one of the twelve apostles of Jesus Christ who according to legend arrived in Spain to preach Christianity.