A section of the A40 has been closed following a serious incident understood to involve a Tesco heavy goods vehicle.
Officers from the Met Police have closed the northbound section of the A40 under the Target Mcdonalds roundabout.
The incident is understood to have taken place around midnight on Tuesday evening.
First aid kits and an AED can be seen in the middle of the hard shoulder on the carriageway.
Specialist road collision specialist have been called to the incident that remains closed to traffic.
It is unclear how seriously injured the person involved is.
The Met have been approached for comment.
