Around 20 firefighters are tackling a blaze close to the National Archives.
London Fire Brigade said two disused water towers which were in the process of being decommissioned caught fire in Kew, south west London, at about 12.30pm.
Both towers are now "100% alight", a fire service spokeswoman said.
A Scotland Yard spokesman said: "No-one is trapped or injured."
Police, who were called at 12.37pm, are providing assistance at the scene, he added.
On its Twitter page, London Fire Brigade said: "The fire in Kew is under control - crews are now tackling remaining hot spots and small pockets of fire. No injuries have been reported."
National Archives UK tweeted: " Thank you for all your concerned tweets. We will update as soon as we can."
The National Archives is the Government's official archive of original documents and records, dating back more than 1,000 years.
National Archives UK tweeted later: " We can confirm the fire has been put out, but we will stay closed to the public in order to recover the building."
Responding to its Twitter followers, the National Archives said "all people and records are safe and were unharmed" following the fire.