Fires in Surrey and London are largely under control, after crews were sent out to battle earlier "weather-related" blazes.
In Surrey, a fire at Hankley Common near Farnham had drawn "several fire engines" to the area, and had earlier been described as a "major incident".
The fire service tweeted: "There is a great deal of smoke so please avoid the area, windows and doors should be closed if nearby and pets kept indoors."
Pictures online showed plumes of smoke rising over the county, with people as far away as Guildford reporting they could see it.
The fire service later tweeted to ask those nearby to stop calling 999 to report smoke clouds, after receiving "an incredibly high" number of calls, adding some local roads had been closed as a result of the incident.
Fire crews in London had been battling wildfires - with blazes in Rammey Marsh in Enfield, Cranford Park in Hayes and Thamesmead. London Fire Brigade (LFB) has described the incidents as "weather related".
In Enfield, close to Epping Forest, LFB said around 100 firefighters were battling the now under-control grass fire, which had grown to the size of four football pitches.
Also under-control is a fire in Hayes, which could be seen from across west London, with the LFB adding 70 firefighters were at the scene. It advised people in the area to close doors and windows.
LFB said crews will remain at the site in an effort to make it safe.
The service also urged people to help prevent further fires by not having barbeques and disposing of cigarettes correctly.
Big Jet TV, which live commentates on planes landing at Heathrow, said that visibility was reduced at the airport, sharing a picture of a smoke covered runway from the nearby fire.
In east London near Newham, another fire is being dealt with in Thamesmead. LFB say 65 firefighters and a fire boat are at the scene close to London City Airport, with those nearby asked to keep doors and windows closed.
It comes after a swathe of wildfires in London and the surrounding area after last week's record temperatures left the ground tinder-box dry.
The UK hit record temperatures on Tuesday, with Coningsby in Lincolnshire reaching a sweltering 40.3C (104.5F) - the first time since records began that the mercury has exceeded 40C in the country.
Some parts of the UK saw significant damage as a result of the extreme heat, such as house and wildfires, melting airport runways and expanding railway tracks.