He snatched the cash box and, with his blazing car out of action, ran off. Police arrived and witnesses pointed them towards the offender.

The suspect was arrested minutes later in a nearby park. Officers managed to put the out the fire in the car, which has now been removed for forensic examination.

The incident happened at about 9pm on Monday night outside the Nationwide building society in Bournemouth, Dorset.

Detective Inspector Craig Travers, of Bournemouth CID, said: "There was a car that had been driven to the scene and parked in a normal manner nearby by the offender.

"The car was deliberately set fire to by the offender, rendering out of use. We don't yet know why he has done this.

"Normally when you get a robbery a vehicle is used to make off from the scene in and swapped for an exchange car later on, at which point the first car is set alight.

"At about the same time of the car fire a man pointing what is believed to have been a handgun approached a Group 4 Securicor officer outside the building society.

"The security guard put the cash box on the pavement and it was picked up by the robber who made off on foot with it.

"A 44-year-old man fitting the description of the offender was arrested a short time later at Horseshoe Common, not far from the scene.

"Officers managed to put the car fire out with an extinguisher. There was some damage to it but it has been removed and is being examined by forensic officers."

Witnesses reported hearing a minor explosion from the car fire at the time of robbery.