The fire at the Cuming Museum and Newington Library in Southwark was reported in late morning, The Guardian reported. Thirty people were evacuated.
Thick smoke blew over the neighborhood as 120 firefighters struggled to contain the flames. The fire was reported to be under control by early evening.
The museum opened in 1906, housing a collection of objects from around the world assembled by Henry Cuming and his father, Richard. In his will, Cuming, who died in 1902, described the collection as "my museum illustrative of natural history, archaeology and ethnology with my coins and medals and ... other curios."
The museum also contains an exhibit on the history of Southwark, a London borough that was the home of the Globe Theater.
Fire officials said the blaze began in the building's roof. There were no reports of injuries, but officials were worried about the collection.
The building was damaged by an incendiary bomb during World War II and did not reopen until 1951.