Four in ten children in primary schools are breathing toxic air that breaches guidelines form the World Health Organisation, an investigation reveals.

A national study shows 3.7 million pupils are being exposed to high levels of pollution on a daily basis - with some of the worst findings among the youngest pupils.

Experts warned that traffic fuelled by the school run is leaving children at increased risk of lifelong conditions including asthma and other breathing difficulties.

The research shows that 42 per cent of all pupils attend schools in in areas which breach limits for the most dangerous particulates - called PM 2.5 - which are largely caused by traffic.

The highest figures were at state-funded nursery schools, where 53 per cent of infants and children are exposed to levels above the recommendations.

At primary schools, the figure is 42 per cent, with a figure of 41 per cent in state-funded secondary schools, with an even higher figure of 52 per cent at independent secondary schools.