The union is lobbying for a legally enforceable maximum temperature of 27°C.
Usdaw general-secretary John Hannett said: 'The fact that there is no legally enforceable upper limit makes it very confusing for responsible employers and allows bad employers to ignore serious health and safety concerns.'
He warned that excessive temperature could lead to heat stroke. 'Employers should watch out for irritability, dizziness, headaches, nausea and fainting,' he said.
A set of guidelines has been produced to help managers make the retail environment bearable for both shoppers and staff.
Key pieces of advice include providing regular cold drinks, relaxing dress codes, improving ventilation and covering heat sources.