A CSP study showed that one in four Brits regularly worked for eight hours or more without breaks, and c-store owners were among the worst.
"Physiotherapists are concerned that overworking and not taking breaks could be costing those who run convenience stores in reduced performance and productivity," CSP professional advisor Leonie Dawson said.
"Standing for long periods is hard on knees, hips or spine, and prolonged sitting can cause pain to the back and hips. Worse still, those who work when they are ill or without a break could be damaging their health in the longer term," she added.
In addition to musculoskeletal problems, poor working practices led to an increased risk of cancer, depression, heart disease and stroke, the report said.
Dawson advised retailers to take regular breaks and alternate between standing and sitting every half-hour.