Police, fire crews, and paramedics are at the scene of the crash, which happened between junctions 9 and 8 at around 10am today (November 25).
The eastbound carriageway is closed to allow the air ambulance to land.
The westbound carriageway is also shut.
Motorists should consider alternative routes.
A police spokesman said: “Officers are currently at the scene of a serious collision on the M20, between junctions 9 and 8.
“Kent Fire and Rescue Service and South East Coast Ambulance Service are also in attendance and the westbound carriageway is closed.”
