The collision occurred on Saturday, 27 February at around 3.35pm on the roundabout at Junction 14 of the M25.
A petrol tanker collided with a white Mitsubishi Lancer that was being pushed in lane one by two occupants. It is believed that the Mitsubishi had broken down on the roundabout prior to the collision.
The driver of the tanker stopped at the scene and assisted police.
It is known that a dark coloured panel van was stationary ahead of the Mitsubishi and the driver may have stopped to assist the occupants prior to the collision. A tipper lorry also overtook the car immediately prior to the collision.
A 44-year-old man is in a life-threating condition and a 20-year-old woman is now in a stable condition.
No arrests have been made.
Officers are appealing for the drivers of these vehicles, as well as any drivers in the vicinity at the time who may have witnessed this or have dash-cam footage, to come forward and help assist with the investigation.
Anyone with information is asked to call the Serious Collision Investigation Unit on 020 8246 9820 and or via 101 quoting Cad 4844/27FEB.