Three in ten child asthma cases in some parts of Britain are caused by traffic pollution, a major international study has found.

The new research reveals hundreds of thousands of cases of the potentially fatal respiratory condition could be avoided with cleaner air.

The study found that, overall, 19 per cent of new childhood asthma cases each year are attributable to nitrogen dioxide pollution.

However, in busy cities the proportion is far higher, with 23 per cent of cases in Manchester due to pollution and 29 per cent in London.

It is thought that pollution from traffic damages airways, leading to inflammation and the development of asthma in children who are genetically predisposed to the condition.

Approximately 1.1 million are receiving treatment for it in the UK at any one time.

The new international research by George Washington University, published in The Lancet Planetary Health, found that the UK had the 24th highest proportion of new childhood asthma cases attributable to traffic pollution of the 194 countries studied.

South Korea topped the list, with nearly one third (31 per cent) of new diagnoses linked to nitrogen dioxide exposure.

"Our study indicates that policy initiatives to alleviate traffic-related air pollution can lead to improvements in children's health and also reduce greenhouse gas emissions," said lead author Dr Ploy Achakulwisut.