Emergency services initially closed the (A316) in Twickenham following the fatal accident, although it has since been reopened.
Chertsey Road was also closed in both directions and severe delays are expected along the Whitton Road Roundabout (B361) all the way to London Road Roundabout (A310).
A London Ambulance Service spokesman said: “We were called at 2:45pm today to Chertsey Road, Twickenham, to reports of a person taken ill in a vehicle.
“We dispatched a number of resources to the scene, including an advanced paramedic, an incident response officer, two medics in response cars and an ambulance crew and we also dispatched our hazardous area response team (HART).
“Sadly, despite the extensive efforts of the emergency services a man was pronounced dead at the scene.”
Congestion is expected to be built up along the westbound route Richmond Circus and along eastbound towards the Hospital Bridge Road as a result of the accident.