The UK's highest court has ruled that the government must take immediate action to cut air pollution.
The ruling represents a significant victory for campaigners, who started legal action after the UK breached EU limits for nitrogen dioxide in the air.
NO2 is produced mainly by emissions from diesel vehicles and is linked to a range of respiratory illnesses.
The Environment Department said work had already been started on revised plans to meet EU targets on NO2.
In a unanimous ruling, a panel of five judges ordered "that the Government must prepare and consult on new air quality plans for submission to the European Commission... no later than December 31 2015".
The case had been brought by ClientEarth, a group of environmental lawyers.
Announcing the decision, Lord Carnwath said: "The new government, whatever its political complexion, should be left in no doubt as to the need for immediate action to address this issue."
Air pollution causes 29,000 early deaths a year in the UK.
ClientEarth had urged a panel of five justices, headed by the court's president Lord Neuberger, to direct the government to produce a new plan for cutting levels of NO2.
Ben Jaffey, representing the group in its case against the Environment Secretary, told the panel at a recent hearing that enforcement by the court was the only "effective remedy" for the UK's "ongoing breach" of European Union law.
A spokesperson for the Department for Environment, Food and Rural Affairs (Defra) said: "Air quality has improved significantly in recent years and as this judgement recognises, work is already underway on revised plans (since February 2014) to meet EU targets on NO2 as soon as possible.
"It has always been the government's position to submit these plans before the end of this year. Meeting NO2 limits is a common challenge across Europe with 17 member states exceeding limits."
ClientEarth has said that under existing plans, some areas such as London, Birmingham and Leeds would not meet pollution limits until 2030. The original deadline for meeting the targets, set in the European Commission's Air Quality Directive, was 1 January 2010.
In May 2013, the Supreme Court declared that the UK had breached its obligations under the Directive.
The case was then heard in the Court of Justice of the European Union (CJEU), which ruled that national courts can force governments to comply with EU rules.
The CJEU empowered the UK courts to take any necessary measure to ensure that the national authority establishes the air quality plan required by the directive.
The legal action returned to the Supreme Court on 16 April this year.
Defra added that the court's ruling had not supported ClientEarth's assertion that the UK had breached Article 22 of the Air Quality Directive by failing to apply for an extension on meeting NO2 limits.