The dusty two-seater, unused since 1960, was in a garage in Gosforth, near Newcastle in northern England.
Only 17 were ever made, and when it's cleaned and auctioned in Paris next month, experts believe it will fetch at least 3 million British pounds ($4.3 million) and possibly much more.
Bugatti once represented the height of motoring achievement. The supercar was so ahead of its time it could go up to 130 mph (209 kph) when most other cars topped out about 50 mph (80 kph).
This particular car is even more valuable because it was originally owned by Earl Howe, a prominent British race car driver, and because its original equipment is intact, so it can restored without relying on replacement parts.
"It has all the finest attributes any connoisseur collector could ever seek, in one of the ultimate road-going sports cars from the golden era of the 1930s," said James Knight, head of the international motoring department at Bonhams, which will auction the car Feb. 7.
Knight and a small number of Bugatti enthusiasts knew of Carr's proudest possession, but not the eight relatives who inherited Carr's estate.
The orthopedic surgeon, who died at age 89, was described by relatives as an eccentric hoarder who never threw anything out. He also left behind an Aston Martin, which was sold, and a Jaguar sports car that was scrapped because it was in such poor condition.
The Bugatti marque is famed for its speed and handling and was a frequent race winner in the 1920s and 1930s. The 57S Atalante was one of its most successful models, each one made by hand with unique details.
The company founded in 1909 by Ettore Bugatti collapsed in the 1940s after a long string of racing victories.
The rights to the legendary Bugatti name were purchased in 1998 by Volkswagen, which has built the Bugatti Veyron, one of the world's fastest and most expensive cars.