Two Charles Darwin manuscripts have been reported as stolen from Cambridge University library two decades after they were last seen.
Staff believed the precious items had been “mis-shelved” within the vast archives late in 2000 and the matter was not reported to Cambridgeshire police until 20 October this year.
The force said it has launched an investigation and notified Interpol.
The two notebooks, including Darwin’s seminal 1837 Tree of Life sketch, were removed from storage to be photographed at the library’s photographic unit, where the work was recorded as completed in November 2000.
During a subsequent routine check in January 2001 it was found that the small blue box containing the notebooks had not been returned to its proper place.
Dr Jessica Gardner, university librarian and director of library services since 2017, said: “My predecessors genuinely believed that what had happened was that these had been mis-shelved or misfiled and they took forward extensive searches over the years in that genuine belief.
“Now we have completely reviewed as a new team what happened and come to a conclusion that that’s not a sufficient position or set of actions to take.”
An appeal for information has been launched on the advice of external experts, including at the Metropolitan police’s arts and antique specialist crime unit, Dr Gardner said. She said there were no leads.
Dr Mark Purcell, the deputy director of Research Collections, said he was confident the manuscripts could not be sold on the open market and it was possible they had “gone to ground”.