Pictures of the scene show huge plumes of smoke rising above the lorry which appears to be in the left lane. The clockwise route of the highway is closed between junction 29 and junction 30, near Grays in Essex. Emergency services are at the scene.
Lorries, vans and cars can be seen queueing behind the blaze.
Essex Fire Service tweeted: “Crews from Grays, Brentwood and Basildon are at the scene of a lorry fire on the clockwise M25 just before Junction 30. We will bring you more information as soon as it is available.”
Witnesses reported that an air ambulance was circulating overhead and looking to land.
One said: ” Air ambulance now circling looking to land.”
Another wrote: ” It circled a few times then stood down.”
Essex County Council’s travel department said the clockwise route had been closed between junction 29 and junction 30.
They said police had told them the closure was likely to remain in place “for several hours”.
The department wrote on Twitter: “UPDATE-M25 Clockwise – CLOSED between J29 (A127) and J30 (A13) due to a vehicle fire. Long Delays.
“The road is likely to be closed for most of the afternoon for cleanup and recovery.
“There are also delays anti-clockwise with onlookers.”
Jamie O’Hara, a former Tottenham midfielder, is among those caught up in the delays.
He said he had been stuck in traffic for hours due to the fire.
He tweeted: “#m25 not moved for two hours apparently bad fire involving a lorry, not looking good.”
The Metropolitan Police’s Roads and Transport unit advised people to “please avoid the area and find an alternative route”.
Reports have suggested a fuel tanker may be involved in the fire.
More to follow.