Some 70 firefighters have been dispatched to battle a blaze on Rammey Marsh in Enfield, north London, as it reissued a warning over barbecues.
Black smoke was seen billowing over the M25 between junctions 25 and 26 according to the London Fire Brigade.
The service claims that despite repeated warnings people have continued to use barbecues while the ground is "tinderbox dry".
A spokesperson said: "This summer has seen an unprecedented long dry spell, with high temperatures so the grass in London is tinderbox dry and the smallest of sparks can start a blaze which could cause devastation.
"Despite our continued warning over the last few weeks, we know there are still people who are barbecuing in parks, dropping cigarettes out of car windows and leaving rubbish lying around."
The brigade said it had received 55 calls about the blaze but the cause was "not known at this time".
Commuters reported on Twitter that three out of four northbound lanes were closed as a result.
The heatwave this week has also seen fires sparked in Mansfield, Notts, Studland Heath in Bournemouth, and in parts of North and South Wales.
On Friday, crews in North Wales were called to a grassland fire in Talacre, Flintshire where a popular holiday park was ablaze.
The fire service said it extinguished the fire close to midnight but that thousands of pounds worth of crops had been destroyed across 200sqm of land.
In the south of Wales residents described watching "ash fall from the sky" as 30 firefighters battled a blaze in St Davids in Pembrokeshire on Saturday.
Roads were closed and smoke filled the sky as local businesses and residents gave out glasses of water to those fighting the blaze, Wales Online reported.
On Twitter stunned Brits shared videos of the chaos.
Footage shared on Saturday also showed Dartford Heath in flames, set alight by the heat as fire crews manned the scene.
In Mansfield, Nottinghamshire, homes were evacuated as images of huge flames across a dry grassy verge by a residential street were shared by local news outlets.
The soaring temperatures have prompted fresh warnings from activists and environmental experts after the hottest day ever recorded last month prompted wildfires in the south.
In July the UK saw temperatures reach a record breaking 40 degrees as it experienced an unprecedented heatwave.
East London was particularly affected with two of the largest wildfires in Wennington and Upminster, while homes and buildings were left destroyed in Dagenham.
Recent satellite pictures shared by NASA show how dried the UK's landscape has become in just a matter of weeks.
The shocking side-by-side images which compare a once luscious green island to a scorched brown landscape have hit hard, with the UK's national drought group calling an urgent meeting.
On Friday environmental officials declared parts of the UK were officially suffering a drought meaning plans to impose a hosepipe ban are imminent.
The areas affected at Devon, Kent, East Anglia and Lincolnshire and water companies will now be forced to action their drought response plans.
