While police were attending the scene a separate car arrived and crashed into a police car, which was attending the original incident, before hitting another lamppost.
The driver of the first vehicle has been taken to hospital for treatment, but nobody else has been injured.
Police are appealing to anybody with information who saw the incident or has video footage.
The A19 southbound near Castle Eden is expected to re-open later this afternoon.
A Durham Police spokesperson said: “We are appealing for information following two road traffic collisions on the A19 near Castle Eden this morning.
“The first is believed to have happened just before 5am when an Audi has collided with a lamppost.
“While officers have been at the scene, a second collision has taken place when a Toyota has collided with a stationary police car as well as a different lamppost.
“The driver of the first vehicle remains at North Tees Hospital with serious injuries and both drivers have been arrested in connection with driving offences.
“No one else is believed to have been injured.
Don’t miss out on the latest news and stories. Subscribe to the Northern Echo by clicking here
Most read
Postcode Lottery wins in Hartlepool, Gateshead and Sunderland
A19 closed both ways near York after “serious collision”
RAF Avro Shackleton arrives at Yorkshire Air Museum
“The southbound carriageway continues to be closed following the incident but is expected to reopen this afternoon.
“Officers would like to speak to anyone who may have witnessed the incident, or the Audi prior to the collision.
“If you have any information or dashcam footage, contact the force’s Collision Investigation Unit by calling 101 and quoting incident reference number 44 of January 12.”