A lorry driver is in a "serious condition" after a collision involving a number of cars at the Willow Tree roundabout in Yeading.
The man was taken to a west London hospital in a serious condition and the driver of one of the other vehicles was also taken to hospital as a precaution.
A Metropolitan Police spokesman said: "It was a collision between a lorry and a number of vehicles and at this early stage it appears that the driver of the lorry had been taken ill prior to the collision."
Police were called at 8.41am on Tuesday morning (June 9) to the roundabout, which is on the A312.
Parents were taking their children to school at the time of the accident.
Kerry Foster, a parent on the school run, saw all three emergency services pass her.
She said: "Traffic was really bad in the area. My usual 10-minute school run took an hour.
"Firemen were there from around 8.40am, turning traffic around on the stretch between Tesco Yeading and White Hart roundabout, so people had to use the outside lane of the bypass to drive in the opposite direction.
She added: "When I came back that way, at around 9.40am, the police had completely shut both sides of the bypass between Tesco Yeading and White Hart roundabout."
All lanes but one were re-opened at 2pm while barrier repairs were being carried out.
The closures were in both directions between the Willow Tree roundabout junction and White Hart roundabout.
Police are unable to confirm how many cars were involved in the collision at this stage.