Campaigners have won a High Court battle forcing the UK government to speed up and improve measures to combat toxic air pollution, which has been breaching European safety limits for several years.
Environmental legal group ClientEarth, which raised the action, condemned measures set out by Westminster to cut harmful fumes as “woeful, inadequate and illegal”.
Official figures suggest air pollution causes at least 40,000 early deaths in the UK every year and costs the country more than £27.5 billion.
The High Court judgment is a second victory for ClientEarth, which won a landmark Supreme Court ruling in April 2015 over the same issue.
Supreme Court justices declared then that immediate action was needed and set a deadline for the government to produce new plans to comply with European Union law on safe limits for nitrogen dioxide in the air, which were to be achieved by 2010.
But Mr Justice Garnham has now declared the current Air Quality Plan legally flawed, saying it “must be quashed” and replaced by an improved version.
He ruled that the environment secretary had failed to take steps to bring the UK into compliance “as quickly as possible” and “fell into error by adopting too optimistic a model of future emissions”.
ClientEarth chief executive James Thornton welcomed the findings and called for a swift solution.
“The time for legal action is over,” he said.
“This is an urgent public health crisis over which the Prime Minister must take personal control.
“I challenge Theresa May to take immediate action now to deal with illegal levels of pollution and prevent tens of thousands of additional early deaths in the UK.”
A spokeswoman for the government’s Department for Environment, Food and Rural Affairs insisted ministers remained committed to tackling air pollution but accepts the court’s judgment. “Our plans have always followed the best available evidence – we have always been clear that we are ready to update them if necessary and have been at the forefront of action in Europe to secure more accurate, real-world emissions testing for diesel cars,” she said.
“We will now carefully consider this ruling, and our next steps, in detail.”
Health and environmental groups hailed the judgment as good news for the public.
James Cant, director of British Heart Foundation Scotland, said: “Our research has shown that air pollution, particularly from small particles in diesel fumes, increases the risk of potentially deadly heart attacks and strokes.”
He continued: “For the 670,000 people across Scotland living with cardiovascular disease, improving the nation’s air quality will help to improve quality of life and protect against further ill health.”
Air pollution activists called for diesel-powered transport to be phased out.
“We know road traffic is the biggest problem for air pollution in the UK and diesel is the worst of all,” said Jenny Bates, of Friends of the Earth.
“To really deal with this air pollution crisis, the government must now introduce plans to phase out diesel from our roads.”
Greenpeace’s Barbara Stoll added: “This judgment matters to every person breathing in the UK but it can only be called a victory when the levels of toxic air actually start to go down.”