The alarm was raised at about 5pm today when customers at the Sainsbury's supermarket in Hedge End, Southampton, Hampshire, began to suffer symptoms including coughing and difficulty in breathing.
The store and a neighbouring Marks and Spencer's shop were evacuated by Hampshire Fire and Rescue Service which scrambled four fire appliances and a hazardous materials unit to the scene.
Of those treated by paramedics, one was taken to hospital as a precaution. It is understood that no one was seriously affected by the gas.
A fire service spokeswoman confirmed that the gas had been identified as a refrigerant.
Fire crews wearing breathing apparatus entered the supermarket before the scene was declared safe and the cordon was lifted.
The spokeswoman said: "We have this evening attended a refrigerant leak at the Hedge End Retail Park on Toolbar Way, Hedge End, Hampshire.
"Hampshire Fire and Rescue Service would like to reassure residents in the local area that there is no public health warning, following earlier reports of a gas leak."
An M&S spokeswoman said that a total of 100 of its staff had been evacuated from the store, and 40 had been treated as a precaution.
She said: "We were advised by police to evacuate our Hedge End store following the detection of a gas leak in an adjacent store. The store remains closed and we are supporting the emergency services."
Shopper Richard Hillier wrote on Twitter: "Hopefully the gas leak wasn't serious, left Sainsburys Hedge End just as emergency services turned up. Haven't stopped coughing since leaving."