Campaigners have won a High Court battle against the Government, forcing them to speed up and improve measures to combat harmful air pollution.
Environmental lawyers ClientEarth took legal action following the "continuing failure to tackle the national air pollution crisis".
A judge at the UK's highest court ruled in favour of their "clean air" judicial review action and declared the Environment Secretary's Air Quality Plan legally flawed - stated that it "must be quashed".
The latest proceedings followed a ruling won by ClientEarth in 2015, in which Supreme Court justices claimed "immediate" action was needed to address the issue.
It is estimated 9,400 deaths occur each year in London due to illnesses caused by long-term exposure to air pollution, while 448 schools in the capital exceed legal air quality levels.
- European Union law on clean air
Limits for nitrogen dioxide were introduced into EU law in 1999, and were due to be achieved in the UK by 2010.
However ClientEarth launched legal action in 2011, saying the 37 out of 47 areas in the UK "remain in breach of legal limits."
- What was the Air Quality Plan?
In April 2015, Supreme Court justices set a deadline for the Government to produce new plans to comply with EU law on limits for nitrogen dioxide in the air.
This was subsequently called the Air Quality Plan, and set out a plan for cleaner air across the UK by 2020, and 2025 for London.
However CleanAir returned to court, claiming the AQP was "flawed", "woefully inadequate" and needs to be "drastically" improved.
On Friday, the judge agreed, and said the Secretary of State "fell into error by adopting too optimistic a model of future emissions".
- What have the Government said?
Prime Minister Theresa May accepted the court's ruling when questioned on the matter in the House of Commons.
She said: "I have been asked about air quality in this chamber previously at PMQs and I have always made clear that we recognise that there is more for the Government to do.
"We have been doing a lot in this area. We have been putting extra money into actions that will relieve the issues around air quality.
"But we recognise that Defra now has to look at the judgment that has been made by the courts and we have to look again at the proposals that we will bring forward."