A High Court ruling is being delivered today on a new legal challenge against the Government over harmful air pollution levels.
Mr Justice Garnham will announce his decision in the latest round of a ''clean air'' battle brought by environmental lawyers ClientEarth over a "continuing failure to tackle the national air pollution crisis".
The judicial review action follows a ruling won by ClientEarth 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 has urged the High Court judge to strike down the "unlawful" plan, and order a new one to be prepared which is "compliant" with EU law.
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''.
During October's hearing in London, ClientEarth argued that unlawful weight was given by the Government to "cost and political sensitivity" when drawing up the new plans.
Nathalie Lieven QC, for ClientEarth, told the judge that the "entire approach" of the Environment Secretary in establishing the 2015 Air Quality Plan (AQP) was "driven by cost", and that was "plainly an irrelevant consideration".
Ms Lieven said the consequences of the "ongoing failure" included a "continuing and significant public health risk", and a related financial cost to the UK economy, "including the cost to the public health system as well as the extensive loss of life and ill-health".
Kassie Smith QC, for the Environment Secretary, told the judge when inviting him to dismiss the challenge: "The assessment of which measures should be included in the plan was based on a thorough and comprehensive assessment of the best available evidence and extensive internal and external consultation.
"The Air Quality Plan and the measures contained in it will achieve compliance in the shortest time possible."
If the plan was quashed it would "do more harm than good" in a situation where the Environment Secretary "has repeatedly made clear that the Air Quality Plan is not static - if appropriate and necessary, the plan may be scaled up, altered, amended etc, in order to address the changing evidence base or other factors".
London Mayor Sadiq Khan, the Scottish and Welsh Ministers, and the transport secretary are all "interested parties" in the case.
A spokesman for the Department for Environment (Defra) has 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."