The UK government freely says that almost all its efforts to cut air pollution in recent years have been driven by EU legislation.

There is one reason why air pollution was a big issue in the London mayoral campaign and why the government is facing a legal challenge on its clean-up plans: the EU’s 2008 clean air directive.

The directive became law in 2010 and set limits for nitrogen dioxide, a toxic gas produced by diesel vehicles. Dozens of UK cities are not forecast to meet the limit until 2020 at the earliest.

The history of Europe’s influence on the air Britons breathe dates back to the 1970s when the UK joined the EEC, with the clean air directive just the latest in a series of EU air quality laws that have tackled air pollution in Britain.

Some factors in driving down certain pollutants were in train without the EU’s influence. For example, sulphur dioxide (SO 2 ) was already declining in the 1970s and 80s as heavy industries closed and coal and oil declined.

But experts say further action was driven by the UK’s membership of the EU. “There is no question that EU air protection legislation has over many years, and still very much continues to, affect measures taken in the UK, leading to air quality improvements,” found a report by the Institute for European Environmental Policy, published in March.

MPs backed that conclusion in March, with the environmental audit committee finding “the EU has led the UK to improve environmental standards in areas such as air and water pollution and biodiversity”.

James Thornton, chief executive of ClientEarth, the environmental lawyers who won a court case against the government over pollution last year, said: “It’s not just Brexit that’s bonkers and ill-considered, it appears that Boris [Johnson] is just as bonkers for wanting us to leave the EEA [European Economic Area]. Quitting the single market would leave the UK without air and water quality laws. Without air quality laws, people will die.”