Four fire engines and around 25 firefighters have been called to a grass fire on Magpie Hall Lane, Bromley.
About an acre of grassland, equivalent to almost a football pitch, was destroyed by the fire.
A London Fire Brigade spokesperson said: "During the hot weather we are attending a large number of grass fires across London.
"We’re asking the public to take care not to drop cigarettes or matches on dry grass and to keep a very close eye on barbecues.
"Dropped glass can also start fires as the sun can magnify the sun’s rays.
"Grass fires can quickly spread and cause serious damage, so if you see grass smouldering, please call 999 immediately.”
• Never leave camp fires or barbecues unattended and extinguish them properly after you have finished using them
• Clear away bottles, glasses and any broken glass to avoid them magnifying the sun and starting a fire
• Dispose of smoking materials such as cigarettes safely
• Explain to children the dangers of playing with and lighting fires
The Brigade was called at 1142 and the fire was under control at 1256. Fire crews from Bromley, Orpington, Biggin Hill, and sidcup fire stations attended the scene.
The cause of the fire is under investigation.