Firefighters have been called to a number of grassland fires in Surrey, London and East Anglia.
A large grass fire on at least eight hectares of land was declared a major incident in Surrey.
The fire service sent several fire engines to Hankley Common, near Farnham.
People have been warned to avoid the area because of the large amounts of smoke and to keep windows and doors closed and pets inside.
Field fires have also been reported in Enfield, Hayes, Thamesmead and the village of Burgate in north Suffolk.
In Surrey, fire crews downgraded the incident on Sunday night but said the wildfire was still ongoing.
People reported seeing smoke over Guildford, Woking and Addlestone and on the M25 near Chertsey.
Firefighters were called just after midday and the fire service has said it expects crews to remain at the scene until at least Monday. The cause is unknown.
Its control room was receiving an incredibly high number of calls and was dealing with a number of incidents. People are asked to only call 999 if they see fire but not to report smoke clouds at this time.
Surrey Police tweeted it was assisting fire crews and had closed Thursley Road. The force asked people to stay away while emergency teams dealt with the incident.
Flying instructor Jordan Bridge said the blaze was one of the largest he has seen in the area.
The 24-year-old, from Alton in Hampshire, said at about lunchtime he saw “huge smoke clouds building to the east of Lasham Airfield” where he works.
“After take-off I could see that the fire was covering a wide area with the smoke extending downwind over Guildford and Woking towards London,” he added.
“Obviously [I was] concerned for all involved in the local area given the already stretched resources of local fire services.”
Crews tackled a blaze on the same stretch of land on Wednesday when four hectares of Hankley Common near Elstead caught fire.
After that incident, the service warned that despite overnight rain, more warm weather was expected and vegetation remained extremely dry, urging the public not to use barbeques and to take care.
Hankley Common – known for its appearance in the Bond film Skyfall – is a heathland habitat and home to ground-nesting birds including nightjars and skylarks. It is owned by the Ministry of Defence and is used for training exercises.
Please avoid the area to allow our crews to work. Stay out of the smoke and far away from the fire as it can travel quickly.
— Surrey Fire & Rescue Service (@SurreyFRS)
Elsewhere, the London Fire Brigade said 15 fire engines and about 100 crew were at Rammey Marsh, Enfield, dealing with a grassland blaze on a 20-hectare site.
Emergency services were called at about 11:30 BST.
About 70 firefighters were sent to Cranford Park in Hayes, London, where five hectares of shrubland was alight.
Dark smoke could also be seen rising into the air in Thamesmead as fire engines, the fire boat and about 65 firefighters dealt with a grass fire in Defence Close.
In Burgate, Suffolk, 18 fire engines have been sent to deal with a stubbled crop fire.
Crews were first called at 12:40 to reports of thick black smoke and the fire service said “the wind is not helping, the fire has been spreading”.
The fire service said it was contained but crews were still trying to put it out.
“Farmers are ploughing firebreaks ahead of the flames,” one witness said.
Norfolk firefighters have been dealing with a number of fires in the open including a crop fire in Stockton.
During the heatwave earlier this week, fire services across the country were under pressure as 15 areas around the UK declared major incidents following multiple blazes.
London had its busiest day since World War Two, the city’s mayor said.