A new legal challenge starts today against the UK government over air pollution levels.
The "clean air" action in the High Court is the latest round of a lengthy battle brought by environmental lawyers ClientEarth.
ClientEarth says it is going back to court to fight the government over its continuing "failure to tackle the national air pollution crisis".
Judicial review proceedings in London follow a ruling won by the law group at the UK's highest court in April 2015.
Supreme Court justices declared that "immediate" action was needed to address the issue, and set a deadline for the government to produce new plans to comply with European Union (EU) law on limits for nitrogen dioxide in the air.
But ClientEarth argues that the Air Quality Plan (AQP) which was subsequently produced is "flawed", "woefully inadequate", and needs to be "drastically" improved.
The campaigning group wants the High Court to strike down the current plan, and order that a new one be prepared.
ClientEarth chief executive James Thornton said, "Defra's latest figures estimate there are 40,000 early deaths across the UK every year because of air pollution.
"The government is acting unlawfully by refusing to turn this situation around. It is failing morally and it is failing legally to uphold our right to breathe clean air. It must come up with far bolder measures, ready to face this issue head-on. Air quality in this country is nothing short of a public health crisis."
Friends of the Earth air pollution campaigner Aaron Kiely said, "The government shouldn't have to be taken all the way to court, again, just to make sure we can all breathe clean air.
"Post-Brexit it is vital that the UK enshrines in law EU legal limits for air pollution, and moves towards complying with World Health Organisation recommended levels in order to save lives."
He said, "Too many children are growing up breathing illegally dirty air, which can lead to impaired lung development for life.
"We know road traffic is the biggest problem for air pollution in the UK and diesel is the worst of all, which is why the government must be bold and put in place a plan to phase out diesel from our roads if it is serious about cleaning up the air we breathe."
Limits for nitrogen dioxide (N02) were introduced by EU law in 1999, and were to be achieved by 2010. ClientEarth, which launched legal action in 2011, says that 37 out of 43 zones across the UK "remain in breach of legal limits".
A spokesman for the Environment Department (Defra) said, "The government is firmly committed to improving the UK's air quality and cutting harmful emissions.
"That's why we have committed more than £2 billion since 2011 to increase the uptake of ultra-low emissions vehicles, support greener transport schemes and set out a national plan to tackle pollution in our towns and cities."
London Mayor Sadiq Khan has joined the legal action.
(Also see: London pollution death toll may hit 2,500 this year: survey).