Police say there are no reports of fatalities, but added that "there could be people trapped" in their cars following the collision about 7.15am today.

A Kent Police spokeswoman said visibility was a problem at the time, and witnesses say there was a "very, very thick" early morning fog hovering over the bridge.

At least eight people have suffered serious injuries, and police say another 200 people are believed to be suffering minor injuries.

Up to 30 medical response vehicles are on the scene and motorists have been warned to avoid the area. Five people have already been taken to hospital.

A statement from the Kent Fire and Rescue Service said: "There are no fatalities but ambulance crews are dealing with a large number of walking wounded casualties.

Motorist Joe Ford says he was a few cars back from the end of the tail-back - and the fog was dense.