The government has been criticised for seeking to delay publishing plans to tackle air pollution until after the general election.
Ministers had been given until 4pm on Monday to set out draft measures on reducing illegal levels of nitrogen dioxide pollution, after the High Court ruled in November that existing plans to meet EU-mandated air quality limits were not sufficient.
But on Friday night, the Department for Environment, Food and Rural Affairs (Defra) lodged an application with the High Court to postpone publication until after June 8.
Air pollution causes 40,000 unexplained deaths in the UK each year and costs the NHS £20bn.
Mayor of London Sadiq Khan told ITV News: "I think it's outrageous that the government is seeking to get an extension from the High Court today, an excuse not to take action to address air quality in London.
"The air quality in London is a killer. More than 9,000 Londoners die prematurely because of the poor quality of air. There are children who attend schools in areas where the air quality is unlawful.
"We've had dithering delay from this government for years now".
There has been speculation the clean air plan could include potentially controversial measures such as charges for motorists to drive diesel vehicles, which cause much of the pollution, in towns and cities, or a diesel scrappage scheme.
Defra has argued a delay is necessary in order to comply with election "purdah" rules on government announcements during the election period.
But critics said the government had been given months to publish the draft plans, after they were ordered to do so by the High Court last November.
Tony Lewis, head of policy at Chartered Institute of Environmental Health said: "They've had months to get their air quality plans ready and using purdah as an excuse is pure political expediency.
"Air quality is a major public health issue and the government's actions show that they are putting politics before our health.
"We hope that the courts throw out the government's application and whoever is in power after June 8, puts a stop to all these delaying tactics and gets on with the job in hand, for the benefit of everyone."
The Prime Minister's official spokesman said: "We remain firmly committed to further improving air quality and cutting harmful emissions."
Speaking in the Commons on Monday, Andrea Leadsom, the Secretary of State for Environment, Food and Rural Affairs, said the government will publish a draft air quality paper by June 30.
Ms Leadsom said the government is "committed to making sure ours is the first generation to leave the environment in a better state than we found it".
"As part of that I am personally deeply committed to the importance of ensuring clean air," she added.
Labour said it would produce an air quality plan within 30 days if it won the election, and legislate for a new Clean Air Act.
The environmental law firm which brought the original case against the government, ClientEarth, said it was considering whether to challenge the application.
Chief executive James Thornton: "This is a question of public health and not of politics and for that reason we believe that the plans should be put in place without delay.
"Whichever party ends up in power after June 8 will need this air quality plan to begin finally to tackle our illegal levels of pollution and prevent further illness and early deaths from poisonous toxins in the air we breathe."
The application has been passed to a judge for consideration, but it appears unlikely a decision will be made on Monday.