Government plans to delay air pollution improvements in 12 UK areas have been refused by the European Commission, which says air quality must improve.
Air pollution reduces average life expectancy in the UK by up to eight months, according to the government's own statistics .
But ministers have been slow to meet agreed European standards on cutting levels of the pollutant NO2.
This comes mainly from vehicles. It causes problems with breathing - particularly for people with heart or lung problems.
The UK has been denied permission by the commission to delay air quality improvements in 12 areas - Aberdeen and north-east Scotland; Belfast; Birkenhead; Brighton; Bristol; Liverpool; Preston; Sheffield; south-west England; south Wales; Swansea and Tyneside.
A judgement will be made at a later date on government plans to delay meeting NO2 standards in major cities until 2020 - or in the case of London, 2025.
London has the worst air of any European capital , and the UK is likely to be fined over the failure.
Air pollution is recognised by the government as the second-biggest public health threat, after smoking. It costs the UK an estimated £20bn a year - that's more than twice the amount estimated for obesity, which gets far more publicity.
Daniel Instone, giving evidence on behalf of Defra, said ministers were considering a nationwide network of low-emission zones in which the most polluting vehicles were banned.
Simon Birkett, a campaigner from Clean Air in London, said the commission's ruling suggested that such a network would now be inevitable.
NO2 pollution affects long-term health. Experts giving evidence to the Environment Committee, EFRA, said the health of Olympic athletes visiting over the summer should not be harmed as long as the UK avoids a heat-induced smog episode.