Clive Robinson from the London Fire Brigade told BBC News the fire had no impact on the main National Archives building
Some 20 firefighters tackled the blaze, which affected two disused water towers at the site in Richmond, south-west London.
London Fire Brigade(LFB), which was called out at about 12:30 GMT, said the blaze was now out.
The National Archives is the official UK government archive and publisher.
Held there are 11 million historical documents of national importance, some dating back more than 1,000 years.
Among its collection are the Domesday Book, parchment, electronic records, photographs, posters, maps and paintings.
A spokeswoman from the National Archives said everyone was evacuated from the building safely and no documents were damaged.