The dangerous levels of air ­pollution endured by hundreds of schoolchildren in a suburb of ­Edinburgh will be made worse by traffic from a planned new Waitrose supermarket, say local residents.

The upmarket retailer is due to submit a planning application later this month for a major new store on St John's Road in Corstorphine. But official figures show that this is already one of the most badly polluted streets in Scotland.

The average level of toxic exhaust gas nitrogen dioxide in the air during 2014 was 60 microgrammes per cubic metre. This was 5% higher than in 2013, 50% higher than the five-year-old legal safety limit and the second highest in Scotland after Hope Street in Glasgow.

Becky Lloyd, a 46-year-old mother of two who lives locally, pointed out that the number of car ­parking spaces for shops would increase from 15 to 140. "I fail to see how bringing more cars and delivery lorries into the area can possibly be anything other than a nightmare for residents," she said.

"To be frank, I find it hard to understand why Waitrose and their developers think that it is a good idea to build a major supermarket and multi-storey car park on a site which has such a problem with ­traffic congestion and pollution."

Helen Crowley, another local ­resident involved in a campaign against the new supermarket, walks her children, aged five and seven, to school past the proposed site. "There are hundreds of ­children walking to and from school via St John's Road every day," she said.

"I was shocked to learn how bad the pollution is and what damage it can do to our health. You see a lot of children at school with asthma inhalers, more than when I was at school, and you wonder if it's due to the pollution."

According to Waitrose's ­developer, Realis Estates, local support for the store has been "overwhelming" but it has ­commissioned an ­independent air quality ­assessment because of the pollution on St John's Road.

"The assessment concluded that our proposed development would have a negligible impact on air ­quality on St John's Road, either during construction or when in operation," said Duncan Mathieson, the developer's managing director.

"We also commissioned a ­transport assessment which concluded that a new Waitrose will not significantly increase the amount of traffic on the roads in Corstorphine and it will provide much-needed shoppers' car parking."