The closure is in place on the westbound entry slip at Junction 2 (A20) and continues along the London-bound carriageway between Junction 3 (M26) and Junction 1 (M25/Swanley Interchange).
Kent Fire and Rescue Service was called at 5.26am to reports of a lorry alight on the M20 between Junction 1 and Junction 2.
Two fire engines attended the scene in Sevenoaks, with crews using breathing apparatus and a main jet hose to extinguish the blaze.
One person was passed into the care of South East Coast Ambulance Service paramedics for smoke inhalation.
National Highways officers assisted by managing road closures during firefighting and recovery operations.
Emergency carriageway repairs are now under way, with traffic being diverted via the M26 westbound to the M25 at Junction 5, before rejoining the M20 at Junction 3 (Swanley Interchange).
Traffic has been reported as coping well throughout the day, but motorists are being urged to allow extra time for their journeys.
National Highways has said the incident is expected to clear between 8.15pm and 8.30pm, with normal traffic conditions likely to return around the same time.
