Erika Delbecque, a librarian at Reading University, found the medieval text buried in a box as she catalogued thousands of items about the history of printing and graphic design the library's archives.
She said: "It's incredibly rare to find an unknown Caxton leaf and astonishing that it has been under our noses for so long.
"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."
Experts called the discovery a treasure worth £100,000 as it was among the first books printed in England by William Caxton's press.
The text is written in Medieval Latin and features blackletter typeface, layout and red paragraph which signify 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, 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."
He continued: "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."
According to the British Library, Caxton was both the first to print a book in English and the first English printer.
