Raiders stole £1m worth of computer games and compact discs when they hijacked a lorry parked off the M40.
The 34-year-old German driver was preparing to get into the truck's sleeping compartment when there was a knock on his window at 1.30am on Friday.
Police said he saw a man in a yellow top and, thinking it was a security guard, opened the door only to be sprayed in the face with a noxious substance.
A brick was hurled through a window and he jumped out of the cab but the keys were in the ignition and the lorry was driven off. The truck, a white Renault tractor unit with a blue-sided trailer with Webber printed on the side, was carrying Playstation games and music CDs worth at least £1m.
The lorry's registration is MSY 2018. Anyone with information is asked to contact police or Crimestoppers on 0800 555111.
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