The Supreme Court has reserved judgement on the UK’s breaches of EU air pollution laws.
The government admits air quality laws will be breached in 15 regions until 2020.
Londoners will have to wait until 2025 for pollution to enter legal limits.
Pollution levels in affected areas are a health risk to people on many of Britain’s busiest streets, especially those with heart or lung problems.
The government says the laws are unrealistically strict.
The BBC understands that it also believes the European Commission is partly to blame because it did not set proper limits on pollution from diesel exhausts in the first place.
The High Court and later the Court of Appeal refused to take action on the issue, ruling that enforcement was a matter for the European Commission.
An environment charity, ClientEarth, will now argue in the Supreme Court that the national courts must enforce EU environment law in the UK.
About 29,000 early deaths each year in the UK are blamed on air pollution – more than obesity and alcohol combined (although air pollution tends to shorten people’s lives by a shorter time).
Pollution from road traffic, particularly diesel fumes, is the most significant cause of poor air quality in most cities. The pollutants of most concern are tiny airborne particles, “PM10s”, and nitrogen dioxide.