Twenty fire engines and around 125 firefighters were deployed to a fire at Somerset House.
Part of the third floor and the roof of the west wing of the building were damaged by fire.
Assistant Commissioner Keeley Foster, who was at the scene, said: "The fire was located in part of the building’s roof space. The age and design of the building proved a challenge for crews as they initially responded. As a result, four of the Brigade’s aerial ladders, including a 64 metre turntable ladder, were deployed to support firefighters as they carried out a complex and technical response. This included creating fire breaks in the roof, which has now limited the spread of flames.
"Around 150 staff and visitors left Somerset House before the Brigade arrived and there are no reports of any injuries.
"Crews have been working incredibly hard through the very hot weather and heat of the fire to protect Somerset House from further damage.
"I’d like to thank all our firefighters for their work so far. Crews will continue to remain on scene today (Sunday 18 August) carrying out further operations.
"Throughout the incident we’ve worked closely with the staff at Somerset House in order to coordinate our response.
"I’d also like to thank the Metropolitan Police Service and London Ambulance Service for their support."
The Brigade's 999 Control Officers took more than 30 calls reporting the fire.
The Brigade was called at 1159 and the fire was under control by 1851. Crews from Soho, Dowgate, Islington and surrounding fire stations attended the scene.
The cause of the fire is under investigation.