Laid out in 2008, the Directive set strict limits for particulate and nitrogen dioxide emissions for all EU member states, with a deadline of 1st January 2010. The deadline could be extended to the 1st January 2015 if the Member State had clear plans in place to reduce emissions over a longer period.

In 2013, following a case brought forward by environmental action group ClientEarth, the UK Supreme Court declared that the UK was in breach of its requirements to reduce nitrogen dioxide emissions by the set deadline. It also has no plans in place to warrant an extension to 2015.

Nitrogen dioxide is produced during high temperature combustion, and is produced by shipping, power generation, industry and household sources, though the majority come from traffic exhaust fumes. The gas is highly toxic, causing ground-level ozone which can lead to premature death through respiratory problems, and also leading to acid rain.

In its report, the EU said air pollution limits were exceeded in 16 zones – Greater London, the West Midlands, Greater Manchester, West Yorkshire, Teesside, Stoke-on-Trent (the Potteries), Hull, Southampton, Glasgow, the East, the South East, the East Midlands, Merseyside, Yorkshire & Humberside, the West Midlands, and the North East.

With reduction plans in place, it added that 15 of these zones would take until 2020 to reach the limits set out for 2010. Greater London, meanwhile, would take until 2025 to reach the original targets.

The UK has two months to respond to the Commission.