Charles Darwin notebooks which were reported as stolen have been returned to Cambridge University Library.

Librarians found the notebooks in a pink bag on Wednesday, March 9, alongside a typed envelope wishing Happy Easter to the staff.

The two notebooks - one of which contains the 185-year-old Tree of Life sketch - went missing from the library in 2001.

Staff believed they were mis-shelved until 2020, when they reported the manuscripts as stolen to Cambridgeshire Police.

The force told Interpol about their investigation, and Cambridge University made a worldwide appeal for information.

The pink bag was left outside the librarian's office on the fourth floor of the building.

It was left in a public area which is not covered by CCTV, and the note was anonymous.

Dr Jessica Gardner, the director of library services who reported the notebooks as stolen to police, said that that the notebooks' safe return is "profound".

Dr Gardner said: "My sense of relief at the notebooks’ safe return is profound and almost impossible to adequately express.

"I, along with so many others, all across the world, was heartbroken to learn of their loss and my joy at their return is immense.

"The sole aim of our public appeal was to have the manuscripts safely returned to our safekeeping and I am delighted to have had such a successful outcome in such a relatively short space of time.

"The notebooks can now retake their rightful place alongside the rest of the Darwin Archive at Cambridge, at the heart of the nation’s cultural and scientific heritage, alongside the archives of Sir Isaac Newton and Professor Stephen Hawking."

She added: "It really is a mystery.

"We don’t know how and we don’t know who."

Dr Gardner said that there have been several changes to the library since 2001, with card-and-pin access to secure areas and extra CCTV.

Charles Darwin studied at Christ's College, Cambridge.

Darwin College on Silver Street is named after Charles Darwin's family, his second son having owned some property where the college now stands.

The 1837 Tree of Life sketch looks at patterns between all organisms and details the scientist's theory of evolution.

Professor Stephen Toope, vice-chancellor of the University of Cambridge, said: "Objects such as these are crucial for our understanding of not only the history of science but the history of humankind."

The objects are due to go on display in July for the library's Darwin in Conversation exhibition.

Cambridgeshire Police confirmed that it will continue the investigation, and has received available CCTV relating to the theft and return of the notebooks.

"Our investigation remains open and we are following up some lines of inquiry," a spokesperson said.

Witnesses or anyone with information can call police on 101 or make a report online (https://www.cambs.police.uk/), quoting reference: 35/71468/20.