The M25 has been closed for more than three hours after a lorry shed its load and spilled oil over the carriageway.
The motorway has been shut clockwise since just before midday (April 19) between junction 23 for the A1 and junction 25 for Enfield.
National Highways says the closure is because of a collision near junction 25 which police and traffic officers have been attending.
Motorway cameras showed debris strewn over the carriageway but the closure has mainly been prolonged by cooking oil that has spilled onto the road and needs cleaning up.
Click HERE for live updates on the M25 closure
The closure has caused delays of more than one hour with the worst of the traffic on the M25 - as of 3.20pm - approaching the closure at junction 23, with traffic beginning to tail back towards junction 21 for the M1.
National Highways tweeted at 2.30pm: "The M25 clockwise remains closed between J23 and J25 following a collision and a large spillage of cooking oil across all of the lanes.
"Contractors are working to remove the oil from the carriageway. Delays of up to 60 minutes on the approach."
Motorists are being diverted off the M25 at junction 23 which is the junction with the A1 and South Mimms services.
Drivers are being asked to follow the hollow triangle diversion symbol which takes them onto the A1 southbound into London and eventually onto the A406 at Henly's Corner near Golders Green.
Motorists are then being asked to follow the A406 North Circular all the way until the junction with the A10 at the Great Cambridge Roundabout, before taking the A10 up to the M25 and rejoining at junction 25.
People should be aware this diversion route is very busy particularly on the A406 North Circular and A10.
Roads in and around St Albans, Potters Bar, Waltham Cross, and Waltham Abbey are also much busier than usual because of the M25 closure.
The M25 is open anti-clockwise but there are delays approaching the incident scene on the other side of the carriageway.
According to National Highways, disruption caused by this M25 closure is currently expected to last into the evening rush hour - around 6pm.
No exact time has been given for when the M25 will fully reopen.
Comments: Our rules
We want our comments to be a lively and valuable part of our community - a place where readers can debate and engage with the most important local issues. The ability to comment on our stories is a privilege, not a right, however, and that privilege may be withdrawn if it is abused or misused.
Please report any comments that break our rules.
Read the rules here