A man has died and a woman is in a critical condition after the vehicle they were in collided with a car during a police pursuit.
The pair were being followed by police on the A50 near Longton in Staffordshire after failing to stop.
Their vehicle collided with a car, whose occupants suffered minor injuries, on the eastbound carriageway at about 06:15 BST, police said.
The road has been closed in both directions.
A spokesman for Staffordshire Police said the police watchdog was investigating.
"Due to our involvement before the collision, we have voluntarily referred ourselves to the Independent Office for Police Conduct," he said.
The collision happened between the Baths Road and Trentham Road junctions.
The occupants of the second vehicle were only taken to hospital as a precaution, the spokesman added.
A spokesperson for the Independent Office for Police Conduct said: "We were notified by Staffordshire Police about this incident earlier today and have deployed investigators to the scene and the police post-incident procedure to assess the level, if any, of our involvement."