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.