"The Tale of Kitty-in-Boots," featuring the exploits of "a well-behaved prime black Kitty cat, who leads rather a double life" was penned by the much-loved children's author in 1914.
But the story never made it into print; Potter had completed the text and begun work on the illustrations when, she later explained, "interruptions began."
Those interruptions -- from the outbreak of World War I to marriage, illness and a growing interest in farming -- meant that the book remained unfinished.
Bundled together with many of Potter's other papers, it was forgotten until Jo Hanks, a publisher at Penguin Random House, read about it in an out-of-print biography.
"There was a mention of a tale about a cat called Kitty, but I didn't know how far she'd got with it, or if she'd intended to publish," Hanks told CNN.
Inspired, she dug around in the archives of London's Victoria and Albert Museum, where many of Potter's papers are kept, and came across what she says was a "lucky find" -- a complete manuscript, a dummy version of the book, and two sketches.
Potter's only known color illustration for the book shows the heroine, Miss Kitty, wearing a tweed jacket, shirt and tie, and carrying a rifle over her shoulder.
So before the story could be unveiled to readers, an artist had to be found to conjure up the pictures which would help bring it to life.
Hanks says there was an obvious choice: Quentin Blake, whose illustrations for Roald Dahl's children's books are almost as famous as the characters themselves.
"Quentin was the first person who sprang to mind; his artistic sensibilities are very reminiscent of Beatrix Potter, and they share the same energy and love of rebellious characters.
"He has really brought Kitty off the page, and I think Potter would have approved of him -- I think they'd have got on very well."
Blake, who chose not to see Potter's original illustration until he had finished his own work on the book, said he had "liked the story immediately."
"It's full of incident and mischief and character ... I have a strange feeling that it might have been waiting for me."
And for long-term fans of Potter's work there's an added extra to look forward to: A special appearance by some old favorites, including Mrs. Tiggy-Winkle and an all-grown-up version of Peter Rabbit.
"Peter is characterized quite differently in this book," explains Hanks. "He's older, rather full-of-himself -- no longer the youngster we knew, getting into trouble -- he's transformed into a rather portly buck rabbit."
"The Tale of Kitty-in-Boots" will be published on September 1, to mark the 150th anniversary of Potter's birth.