Road users are advised that the M25 is closed anti-clockwise between junctions 23 and 21a, following a serious incident this afternoon.
A lorry carrying cooking oil and foodstuff has overturned, spilling its content, which later caught fire, onto the carriageway.
Emergency services have been at the scene since 3.20pm.
Structures engineers are currently on site assessing damage to the road and overhead gantry. A police investigation is underway. The road will be reopened as soon as it is safe to do so but the closure is expected to last for several hours, and during rush hour.
The M25 is closed anti-clockwise between junction 23 (access to the A1(M)) and junction 21a (access to the M1).
Road users wishing to use the M25 anti-clockwise need to exit the M25 at junction 23 and take the A1(M) north; exit at junction 3 and join the A414 west. Take the A405 south and rejoin the M25 at junction 21.
Drivers are advised to plan their journeys, allow extra time to travel and avoid the area if possible.
The Highways Agency provides live traffic information via its website (www.highways.gov.uk/traffic), local and national radio travel bulletins, electronic road signs and mobile platforms, such as the iPhone.