Asda said it would start selling blue reliever inhalers over the counter without prescription from Tuesday.
Customers over 16 will be able to buy two inhalers for £7.
The Department of Health said that medicines must be dispensed by qualified staff "in line with all legal requirements".
In England such items would incur a charge of £7.65 if obtained on prescription; in the rest of the UK prescription charges no longer apply.
There are two different types of inhalers, "relievers" and "preventers", available to the 5.2 million people in the UK who suffer from asthma.
Blue reliever inhalers - the ones Asda will be selling - contain the drug salbutamol and are used if someone is feeling wheezy or suffering an attack.
Preventer inhalers are taken twice daily to help keep asthma under control.
Faisal Tuddy, deputy superintendent pharmacist at Asda, said the service was designed to be easy and convenient but would be closely monitored.
"All of our pharmacists have been trained and know they mustn't sell the inhalers without due care," he said.
Dr Samantha Walker, executive director of research and policy at Asthma UK, said the scheme was interesting in principle but she was uncertain how it would work in practice.
She said: "We applaud anything that is going to help asthma suffers but this new service has raised a few questions for us.
"Our main worry is people will overuse their inhalers when they know this service is available. If you use your inhaler too much you may end up in hospital."
Customers will be limited to two inhalers at a time which will have to last them eight weeks.
They will need to fill in a questionnaire about their condition before buying the medication.
Qualified pharmacists and an online doctor service will oversee the sale of the inhalers.
A Department of Health spokesman said: "Medicines should be dispensed by appropriately qualified staff and in line with all legal requirements."
Asthma inhalers will go on sale in supermarkets for the first time this month, allowing sufferers to get the medicine without seeing their doctor.