A grey BMW 335D was travelling northbound on the A15 at about 6.45pm on Friday when it came off the road at the Paston Parkway roundabout, at the junction with David’s Lane, and crashed into a tree.
The driver, a man in his 30s from Whittlesey, was taken to Peterborough City Hospital where he remains with serious injuries.
The back seat passenger, a woman in her 30s from Whittlesey, suffered life-threatening injuries and remains in the same hospital in a critical but stable condition.
The front seat passenger, a man in his 30s from Peterborough, received minor injuries and did not require hospital treatment.
Officers are now looking to speak to anyone who witnessed the collision or the car just prior. They would particularly like to hear from the driver of a red Ford Fiesta who may have been in area at the time of the crash.