The incident was reported at 8.29am today (November 25), between junctions 9 and 8.
Kent Fire and Rescue Service and the South East Coast Ambulance Service are also at the scene.
The Londonbound carriageway will remain closed into the afternoon.
The coastbound carriageway has reopened, but traffic is moving slowly as tailbacks clear.
A Kent Police spokesman said: “Officers are currently at the scene of a serious collision on the M20, between junctions 9 and 8.
“The incident involving a car, lorry and HGV was reported at 8.29am on Tuesday 25 November 2025.
“Kent Fire and Rescue Service and South East Coast Ambulance Service are also in attendance and the Londonbound carriageway will remain closed into the afternoon.
“The coastbound carriageway has now reopened and traffic is moving slowly while tailbacks clear.”
