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.
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The attack is said to have happened in Basingstoke on Monday.
Wednesday morning's weather for the west of the region
Wednesday morning's weather for the east of the region