Two of Charles Darwin's notebooks containing his pioneering ideas on evolution and his famous "Tree of Life" sketch have been returned anonymously after going missing for 21 years, Cambridge University Library said on Tuesday.

The prized documents were left on the floor in a public area of the British university's library in a pink gift bag, with a typed note wishing a Happy Easter to the librarian.

The manuscripts are in good condition and with no obvious signs of damage, the university said in a statement.

The British scientist filled the leather notebooks in 1837 after returning from a pioneering voyage on the ship HMS Beagle. The library said they were worth millions of pounds.

In one book, Darwin drew a diagram showing several possibilities for the evolution of a species and later published a more developed illustration in his 1859 book "On the Origin of Species".

"My sense of relief at the notebooks' safe return is profound and almost impossible to adequately express," said university librarian Jessica Gardner, who raised the alarm in 2020 that the notebooks were feared stolen.

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

Cambridge University Library first listed the notebooks as missing in 2001, after they were moved out of a strong room for special collections to be photographed.

They were believed to have been incorrectly refiled elsewhere in the vast building, which contains around 10 million books, maps and manuscripts and has one of the world's most significant Darwin archives.

However a major search in 2020 -- the largest in the library's history -- failed to locate the notebooks.

"Curators have concluded the notebooks... have likely been stolen," the library said at the time.

The library called in police and the books were listed on Interpol's database of stolen artworks, called Psyche.

The notebooks will go on display over the summer "to give everyone the opportunity to see these remarkable notebooks in the flesh", Gardner added.