Hertfordshire Police and Hertfordshire Fire and Rescue Service are in attendance at the lorry that caught fire between J18 and J19.
The fire sends thick smoke into the sky, as seen in the pictures.
By 8am, delays in traffic reach 60 minutes.
The service urged motorists to take care as they approach the scene, and allow extra time for their journey.
In an update on Twitter, National Highways said: “There are currently 3 (of 4) lanes closed on the #M25 clockwise between J18 and J19 due to a vehicle fire.