Thousands of children are playing in school playgrounds near roads with harmful levels of illegal air pollution, new analysis suggests.
The findings are part of a campaign by environmental lawyers that claims more than 950 schools are near roads which suffer from harmful pollution levels.
Environmental law charity ClientEarth, which is behind the "poisoned playgrounds" campaign that seeks to protect children from dirty air, bases its findings on an analysis of the latest government data.
Billboard adverts are going up in some of the most polluted towns and cities, including Birmingham, Cardiff, Derby, Glasgow, Leeds, London, Manchester and Southampton, with details of the number of schools in the area near polluted roads.
Supporters of the campaign are being urged to put pressure on the government, via their MPs, to take action to curb air pollution, which causes problems such as heart and lung diseases and asthma and affects children's development.
Air pollution is linked to the early deaths of about 40,000 people a year in the UK.
ClientEarth air quality lawyer Alan Andrews said: "Thousands of children in this country are playing in playgrounds near illegally polluted roads.
"This is a legal and moral failing of our political leaders that puts children's health at risk at a time when they are still growing and therefore vulnerable.
"Naturally, parents want something to be done. So do we."
A government spokeswoman said: "We have put in place a #3 billion plan to reduce emissions and improve air quality.
"As part of that investment, we are supporting local authorities to develop plans to address pollution hot spots in their areas, including near schools.
"Next year we will publish a comprehensive Clean Air Strategy tackling all forms of air pollution."