The Government will have to do more to tackle illegal levels of air pollution after a High Court judge ruled its current plans are "unlawful".
Mr Justice Garnham told the London court on Wednesday the approach to tackling pollution in 45 local authority areas was "not sufficient".
The ruling is the third victory against the Government for environmental lawyers from ClientEarth over proposals for reducing levels of harmful nitrogen dioxide.
The court previously heard that, eight years after the UK was found to be in breach of legal limits on the pollutant, levels were still too high in 37 out of 43 zones across the country.
ClientEarth won two previous rulings against ministers over the levels, forcing the Government to draw up new plans last year for reducing nitrogen dioxide, much of which comes from vehicles, to within legal limits.
The measures included £255 million to help local authorities come up with ways to improve air quality, and proposals to end the sale of all conventional petrol and diesel cars by 2040.
Its latest case was brought against the Environment Secretary and the Transport Secretary, while a claim against the Welsh Government was discontinued after it agreed to work with ClientEarth on new proposals.
Mr Justice Garnham said the Government's plan was "seriously flawed" in respect of 45 local authority areas which are not expected to reduce pollution to within legal levels until 2021.
He said: "Because the obligation is zone-specific, the fact that each of the 45 local authority areas will achieve compliance in any event by 2021 is of no immediate significance.
"The Environment Secretary must ensure that, in each of the 45 areas, steps are taken to achieve compliance as soon as possible, by the quickest route possible and by a means that makes that outcome likely."
The judge rejected ClientEarth's case in relation to the Government's monitoring and modelling and its plans for five cities, including London, which are not expected to reach compliance until 2028.
He said the Government's approach to the five cities was "sensible, rational and lawful".