London Fire Brigade on Tuesday declared a major incident as firefighters tackled blazes across the British capital during a record-breaking heat wave.
The city's mayor, Sadiq Khan, said there had been a "huge surge" in fires, putting the service under "immense pressure".
In one incident, 15 fire engines and some 100 firefighters were called to a huge fire in Wennington on the outskirts of east London.
Television footage showed thick black smoke billowing into the air, and houses and fields on fire being doused with water.
A fierce heatwave sweeping western Europe has pushed temperatures up to record levels in parts of south, southeast and eastern England.
At Heathrow Airport, west London, a new record was set of 40.2 degrees Celsius (104.4 degrees Fahrenheit).
LFB urged people only to call in an emergency, not to have barbecues or bonfires, dispose of cigarettes safely and not leave bottles or broken glass on the ground.
"Even the smallest sparks could cause a fire," the brigade's assistant commissioner for operational resilience, Patrick Goulbourne, said.
Elsewhere around the country, fire services were tackling hundreds of wildfires caused by the extreme temperatures turning fields tinderbox dry.
Charles Spencer, the brother of the late Princess Diana, tweeted a photograph of wildfires visible from the family's Althorp estate in Northamptonshire, central England.