Two Charles Darwin manuscripts have been belatedly 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 the year 2000 and the matter was not reported to Cambridgeshire Police until October 20 this year.
The force said it has launched an investigation and notified Interpol.
It is difficult to estimate the value of the notebooks given their unique nature but it would probably run into many millions of pounds, the library said.
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 box containing the notebooks had not been returned to its proper place.
Cambridge University Library has more than 210 km of shelving and is home to around 10 million books, maps, manuscripts and other objects.
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.
There have been continuous searches since the notebooks went missing, she said, and it is now thought "likely that theft occurred".
Dr Gardner said that security policy was different 20 years ago, adding: "Today any such significant missing object would be reported as a potential theft immediately and a widespread search begun."
"I'm heartbroken," she added.
"We've devoted the whole of our careers to the preservation of cultural heritage and we're devastated by what has happened."
The two Darwin notebooks had previously been digitised and their content is available online.
Professor Stephen J Toope, vice-chancellor of the university, said: "Cambridge University Library is one of the world's great libraries and home to globally important collections, assembled and cared for over six centuries, and encompassing thousands of years of human thought and discovery.
"As a result of this appeal for help, we hope to locate the missing Darwin notebooks and restore them to their rightful place alongside the University Library's other treasures, making them available to scholars and researchers in the centuries to come."
Australian Associated Press
