More than 225 firefighters have been tackling a grass fire the size of about 150 football pitches.
Police warned people to avoid the Wanstead Flats, in Epping Forest, north-east London, due to “very heavy smoke” coming from the “tinder-dry” grasslands on Sunday.
The London Fire Brigade (LFB) said the 100-hectare (one square kilometre) blaze was being tackled by firefighters using 40 engines but said there is no immediate risk to property.
LFB group manager Rob Davies said: “Keep windows and doors shut, a lot of smoke is drifting across roads and while there is no immediate threat to property you should avoid the area if you can.
“Crews are working very hard at the scene and are using a variety of hose reels, water packs but also beaters to tackle the fire.”
Images from a police helicopter showed smoke rising from the scorched field.
Metropolitan Police from a ward neighbouring the fire tweeted: “Very large fire on the tinder-dry #WansteadFlats. Very heavy smoke will affect roads in the area.”
The LFB said the brigade was first called at 4.05pm and received more than 110 calls over the “very visible” fire.