The Government must take steps to tackle air pollution within days or face further legal action, it has been warned.
Environmental law firm ClientEarth has sent a final warning letter to Environment Secretary Liz Truss, giving her 10 days to act or face action in the High Court.
The group, which won a ruling against the Government in the Supreme Court over its failures on air pollution last year, says the Environment Department's (Defra) recently published plans to cut pollutants fall "woefully short" of what the court ordered.
The move comes after a report from the Royal College of Physicians and the Royal College of Paediatrics and Child Health warned an estimated 40,000 people die early each year in the UK because of air pollution.
European rules set limits for key pollutant nitrogen dioxide (NO2) which should have been met by 2010, but ClientEarth said the plans which the Government was ordered to produce by the Supreme Court do not see the UK meeting legal targets until 2025.
The "air quality plan" published late last year focused on bringing in clean air zones in five English cities by 2020, in which the most polluting buses, taxis, coaches and lorries will be charged to enter the centre.
But the move does not cover private cars, which ClientEarth said are one of the biggest sources of poor air quality in cities.
London, which breached its air pollution limits for NO2 for the whole of 2016 just a week into January, is set to introduce a ultra-low emission zone in London by 2020 which will cover all vehicles, but is not expected to meet legal levels until 2025.
ClientEarth chief executive James Thornton said: "Despite an order from the UK's highest court, despite tens of thousands of premature deaths in this country every year and despite clear evidence to show that air pollution has a terrible effect on the health of vulnerable groups like children, the Government has consistently ducked its responsibility to ensure our right to clean air.
"We have had to issue this legal warning to the Government because of its failure to produce a plan that would bring air pollution down as soon as possible."
The law firm said it had asked the Government to produce new plans with a list of measures that will bring air pollution down to within legal limits in the shortest time possible and put the proposals out to consultation.
ClientEarth said that if it does not receive a satisfactory reply within 10 days, it will launch legal proceedings in the High Court.