A police car was clipped by a HGV while dealing with a three-vehicle incident on the M25 near Potters Bar.
A police spokeswoman said that one vehicle had run out of fuel and broken down on the anticlockwise carriageway near Potters Bar, following which two other motorists had stopped to help.
A police car then also pulled up, but while at the scene was clipped by a passing HGV.
The incident was initially thought to be a three-vehicle crash but that later emerged not to be the case, the spokeswoman added.
Highways England said that following the incident, which began at around 11am, three lanes were closed and Herts Police and traffic officers were at the scene.
The police spokeswoman said nobody was injured but officers are still dealing with the incident near Junction 24 (Potters Bar).
It has caused long delays and motorists are being advised to plan ahead before using the route.
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 The ability to comment on our stories is a privilege, not a right, however, and that privilege may be withdrawn if it is abused or misused.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