Update: latest photos and video from Dartford Heath to Wennington as fire breaks out either side of The Thames near London
Some homes are being told to evacuate immediately as emergency services seek to get the blaze under control.
London Fire Brigade has declared a major incident due to "a huge surge" in blazes across the capital amid the 40C heat.
It comes as around one hundred firefighters tackled a huge blaze in the village of Wennington, east London, on Tuesday afternoon, with television footage showing black smoke billowing into the air, with buildings and fields on fire.
Sadiq Khan said LFB is under "immense pressure" thanks to a number of fires across the city.
The London Mayor said on Twitter: "London Fire Brigade has just declared a Major Incident in response to a huge surge in fires across the capital today.
"This is critical: @LondonFire is under immense pressure.
"Please be safe. I'm in touch with the Commissioner and will share updates when I have them."
It came as temperatures in the city hit around 40C amid the heatwave blasting the nation.
LFB said 15 fire engines and around 100 firefighters were called to the scene in Wennington at just after 1pm.
At least one home appeared to be completely destroyed in the blaze, while smoke covered a wide area.
Nearby fields were seen to be scorched.
The fire brigade said on Twitter: "We've got 15 fire engines and around 100 firefighters dealing with this blaze on The Green in #Wennington."
It comes as Temperatures have reached 40C for the first time on record in the UK, with 40.2C provisionally recorded at Heathrow Airport, the Met Office said.
The threshold was hit at 12.50pm as much of the UK sweltered in a heatwave, with parts of England and Wales under a red warning for extreme heat, posing a danger to life, pressure on the NHS and disruption across transport networks.
The extreme heat is fuelled by climate change, which is making every heatwave more intense, frequent and likely, scientists warn.
The new high for daytime temperatures came after the UK experienced its warmest night on record on Monday, with temperatures remaining in the mid-20s.
The 40.2C recorded at Heathrow beats the previous record for the UK of 38.7C in Cambridge three years ago, by 1.5C, and the Met Office warned temperatures were still climbing early on Tuesday afternoon.
Charlwood, Wisley and Chertsey in Surrey, and Kew Gardens and Northolt in west London were among the places to exceed the 2019 record by early afternoon, with temperatures climbing above 39C.
More will follow on the breaking incident...