A unique example of medieval printed text by pioneer William Caxton, which had once been used to reinforce the spine of a book, has been hailed as a "thrilling" find.
The two pages from a priest handbook dating back to late 1476 or early 1477 was found buried in a box at Reading University's archives by librarian Erika Delbecque as she catalogued thousands of items about the history of printing and graphic design.
The treasure was among the first books printed in England by Caxton's press and could fetch £100,000 if it went to market, experts suggest.
Ms Delbecque, who said she suspected it was "special as soon as I saw it", said it was "incredibly rare to find an unknown Caxton leaf and astonishing that it has been under our noses for so long".
She said: "This well-preserved item is the only one of its kind, and one of just two surviving fragments from this medieval Caxton book in existence.
"The leaf had previously been pasted into another book for the undignified purpose of reinforcing its spine. We understand it was rescued by a librarian at the University of Cambridge in 1820, who had no idea that it was an original Caxton leaf."
It is written in medieval Latin and features blackletter typeface, layout and red paragraph that marks it out as an example of very early western European printing.
No other copies of the pages, printed either side of a single leaf of paper, are known to have survived.
Early printing specialist Andrew Hunter, of Blackwells Books, who carried out the valuation, said "the discovery of even a fragment from among Caxton's earliest printing in England is thrilling to bibliophiles, and of great interest to scholars.
"If this were ever to come on the market, there would definitely be competition for it. It would be a great prize for a private collector, and a feather in the cap of any institution."
Caxton was both the first to print a book in English and the first English printer, according to the British Library. He realised the commercial potential of the new technology while working as a merchant in the Low Countries and Germany. Caxton set up his own printing press in London in late in 1475 or early in 1476.
The find is from a book called the Sarum Ordinal or Sarum Pye which helped priests to prioritise religious feast days for English saints.
It was part of a collection that previously belonged to late typographer John Lewis and his wife Griselda, a writer and book designer. It was bought by the university for £70,000 at auction in 1997 with help from the Heritage Lottery Fund.
The leaf then lay among many thousands of other items in the archives before being identified, according to Reading University.
The only one other surviving fragment of the book is held at the British Library in London.
Caxton expert Dr Lotte Hellinga, a former deputy keeper at the British Library, said: "It is very rare that an unknown piece of printing by William Caxton is brought to light. The example found in Reading belongs to a different part of the book than those held in the British Library.
"Its condition is good, considering it spent some 300 years bound in the spine of a book and another 200 resting forgotten in an album of fragments rescued from other bindings."
The new find will go on display at Reading University's Merl museum on London Road from May 9 to May 30.