Air pollution is the fourth biggest public health risk in the country, alongside cancer, obesity and heart disease, the Prime Minister has admitted.

Replying to a letter signed by 220 doctors, warning that “time is running out” to deal with the UK’s “toxic air scandal” Theresa May also admitted: “It disproportionately affects some of the most vulnerable in our society, including the elderly, people with lung and heart conditions, and the very young.”

The letter states that children’s lung growth is being stunted by toxic pollution, which is leading to other health problems, notably asthma.

In her letter, the Prime Minister blamed diesel vehicles as a major cause of the problem. Diesel cars received subsidies by the Labour government, on the basis that they emit less carbon dioxide than petrol-powered cars, but it is now known they emit other harmful pollutants, known as nitrogen oxides. It has also since been revealed their levels of emissions were covered up by Volkswagen, in a major scandal.

Emphasising the Government’s determination to tackle the problem, the Prime Minister said: “Poor air quality is the fourth largest risk to public health, behind only cancer, obesity and cardiovascular disease.

“It disproportionately affects some of the most vulnerable in our society, including the elderly, people with lung and heart conditions, and the very young.”

The Prime Minister has been urged to begin phasing out diesel vehicles, but motorists who were encouraged to buy them by the government are now very angry that new incentives to discourage their use has rendered their cars worthless.

A recent study by the London Mayor’s office linked toxic air pollution to 9,000 deaths a year.

Replying to Professor Jonathan Grigg, Professor of Paediatric Respiratory and Environmental Medicine, Queen Mary University of London, the Prime Minister added: “I agree with you that one of the main reasons our cities continue to face pollution problems is the significant levels of NOx (nitrogen oxides) emissions that diesel vehicles produce.”

“Harmful emissions from transport contribute significantly to the air quality challenge we face.”