Emergency services have been called the M4 motorway northbound between junctions 4 and 4A near Heathrow airport following a collision.
It is understood that a person has been struck whilst walking on the live carriageway.
Traffic officers from the Met Police, London Ambulance service and Fire crews have all been mobilsed to the incident that happened just on Thursday afternoon. Traffic has been brought to a standstill.
All three lanes of the motorway have been closed to traffic. The closure is likely to be in place until at least 4pm
A spokesman for Highways England said: The M4 Heathrow Spur road is closed northbound between J4a and J4 due to a serious incident. Met Police are leading the incident and all other emergency services are on scene including the London Air Ambulance.
Traffic is also being held on the southbound spur road whilst the air ambulance is on scene. Once it departs the incident southbound traffic will be released.
Traffic is advised to divert on to local roads in the area to bypass the incident.