A masked raider went into a Hemel Hempstead bookies on Sunday evening, pulled out a gun and ordered that the worker there hand over cash.
The terrifying attack happened at Ladbrokes in Marlowes at around 7.15pm.
Hundreds of pounds was taken from the till, which the gunman forced his victim to stuff into a bag before he made off with the haul.
He is described as black, aged from his late 30s to early 40s, 5ft 7in to 5ft 9in tall, of medium build. He was clean shaven and wearing a dark jacket with the hood up, a balaclava and dark trousers.
The ordeal has prompted one worker, who is employed at another local bookmakers, to call for better security measures at betting shops.
The man, who does not want to be named for fear of losing his job, said: “It is rife within the industry now and it is getting quite serious because they know that many of these bookmakers are single manned in the evenings.
“A lot of them have this single manning policy where people are left on their own to fend for themselves.
“If you are held up at gunpoint it is a traumatic situation and these people are going off sick.
“There is a lot of unhappiness about single manning because it is simply for the profits.”
Anyone with information about the incident is asked to contact Herts Police on the non-emergency number 101 quoting crime reference D1/13/1748. Alternatively, call Crimestoppers anonymously on 0800 555 111.
Dacorum Chief Insp Glen Channer said: “We are determined to find the person responsible for this.
“It was a very distressing robbery for those involved however, I’d like to reassure local residents that these kinds of crimes are extremely rare in Hertfordshire.”