The man is described as being in his 50s, of athletic build, about 5 feet 8 inches tall and was wearing a grey or white hoodie, black jeans, a dark coloured puffer jacket and gloves. He spoke with a local accent.
Four members of staff and the same number of customers were in the shop at the time of the incident.
The man escaped with a quantity of cash a few minutes later and was seen running towards a supermarket via Angel Walk and Angel Lane. Police believe he may have had a getaway car waiting for him.
More top news
Friday's forecast for the west of the region from Martin Stew.
Martin Stew has the latest weather forecast for the east of the region.
Martin Stew has the lunchtime forecast for the west of the ITV Meridian region.