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.
Pictures on social media show people getting out of their cars as all lanes are at a standstill.
Highways England have said the road could be shut for three hours.
More to follow…