A suspected gunman robbed a security van in a Marks and Spencer car park and police are appealing for information.
The man, dressed in a grey tracksuit, approached the van with what is believed to be a handgun.
He threatened the driver, stole a cash box and escaped in a silver Nissan Primera.
It happened at the back of Marks and Spencer in New Road, Princes Risborough, just before 11am on Monday morning. The victim was not injured.
Police would like to speak to anyone who has any information about what happened.
The suspected gunman is aged in his 20s, 5ft 10ins tall and slim.
Call 101 or Crimestoppers anonymously on 0800 555 111.