A third of all children living in Britain, around 4.5 million, live in areas with harmful air pollution, a report by the UN children's agency UNICEF said.
Britain's government last month announced plans to curb air pollution after the European Commission said it would take Britain and five other European Union member states to the EU Court of Justice for failing to respect air quality limits.
UNICEF said the plans do not go far enough and called for more funding for measures to help reduce pollution in the worst affected areas.
"Under the government's air quality plans, unsafe levels of air pollution in around 71 percent of UK towns and cities is likely to be the case for another 12 years," the report said.
A separate report by Global Action Plan, also released on Thursday, found that children are 30 percent more exposed than adults to dangerous pollution coming from vehicles on busy roads due to their shorter height.
British environment minister Michael Gove said the Global Action Plan research was troubling, and showed why the government needed to take strong action to improve air quality.
"Our new Clean Air Strategy sets out how we will be the first major developed economy to reduce air pollution in line with World Health Organization (WHO) limits and we have invested 3.5 billion pounds ($4.6 billion) to reduce harmful emissions," he said in a statement.