Two police officers were hurt, one seriously, when their patrol car and a suspected getaway vehicle were involved in a head-on smash.
The officers were trying to stop a man suspected of holding up a convenience store in Felpham, near Bognor, when the two-car smash happened.
One officer has been kept in hospital with serious injuries. The other had minor injuries and was released after treatment.
The robber had threatened the owner of the shop in Felpham Road, hinting that he had a firearm. He fled with cash and jumped into a vehicle.
The shop owner alerted police and the two officers spotted the suspect in Walberton.
They were trying to turn their squad car around when the accident happened, at 8.45pm on Sunday.
Detective Constable Neil Gilliard said there were a number of people outside the shop when the robbery happened.
He has appealed for anyone with information to call Sussex Police on 0845 6070999.
A man has been arrested and charged with robbery, aggravated vehicle taking, theft, driving while disqualified, drink-driving and driving while uninsured.
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