An air ambulance has landed on the motorway following the incident, by Junction 5 for the M26. A motorbike is believed to have been involved. Motorists report extremely long queues of traffic as emergency services rush to the scene.
Both carriageways have been shut.
Highways England have said Kent Police and Traffic Officers are currently at the scene of the “serious collision”.
They said the road could be shut for three hours.
Travel service Inrix says: “M25 anticlockwise blocked, stationary traffic due to accident and serious accident, a motorbike involved at J5 M26.
“The motorbike has crashed after the clockwise exit slip to the A21 Southbound – this slip road runs parallel to the M25 anticlockwise.
“Traffic has been stopped while the air ambulance attends.”
The link road between the M25 J5/M26 will remained closed while an investigation is carried out.
Pictures on social media show people getting out of their cars as all lanes are at a standstill.
One person shared a picture on Twitter showing an ice cream van serving customers while all the cars are stationary.
They wrote on Twitter: “Everyone avoid the m25 right now only good thing is there’s an ice cream serving.”
Another person said: “I have now been stuck on a closed M25 for an hour. I’m BORED and am worried that eventually I’ll need to wee.
“Rumours of road being closed until 7pm making me anxious….”
Traffic officers started to disperse the traffic shortly before 4.30pm.
A video from one motorist shows people rushing back to their cars as vehicles start to move in front.
But one driver, who was travelling anti-clockwise, said traffice was still at a standstill.
They wrote: “Still at a standstill just past Clackets going anti-clockwise, however no cars are coming Clockwise, we want to get home!!”