Cleveland Police say the Independent Office for Police Conduct are investigating after a car crashed during a police pursuit.
The man who was being followed along the A66 by police was driving a suspected stolen car.
He was seriously injured when the car left the road on Sunday morning at around 11:20.
The man in his 20s is receiving treatment in hospital for his injuries.
The silver C Class Mercedes which had been reported earlier as suspected stolen was being pursued by Cleveland Police after the force had been notified it was travelling in the area.
As is routine in these circumstances, Cleveland Police have informed the Independent Office for Police Conduct which has begun inquiries into the circumstances surrounding the incident.