Four fire engines and around 25 firefighters tackled a blaze at a disused leisure centre in Central Parade, New Addington.
An electrical intake room was destroyed by fire and a water tank room on the first floor was also partially damaged. There were no reports of any injuries.
The Brigade was called at 2013 and the fire was under control by 2138. Fire crews from Addington, Biggin Hill and Woodside stations attended the blaze.
The cause of the fire is under investigation by the Metropolitan Police.