The UK government may face legal action after seeking to delay publishing its plan to tackle air pollution until after the general election.
The courts had given ministers until 16:00 on Monday to set out draft measures to combat illegal levels of nitrogen dioxide (NO2) pollution.
A 2016 court ruling said existing measures proposed by the government did not meet the requirements of law.
The general election is scheduled to take place on 8 June.
In a surprise move late on Friday, the Department for Environment, Food and Rural Affairs (Defra) lodged a fresh application with the High Court to postpone publication of its draft clean air plan until after the election.
It argued to move was necessary in order to comply with election "purdah" rules limiting government announcements with political implications during the election period.
The environmental lawyers who have brought legal proceedings against the government, ClientEarth, said they were considering whether to challenge the application.
"The unacceptable last minute nature of the government's application late on Friday night, after the court had closed, has meant that we have spent the weekend considering our response," said chief executive James Thornton.
"We are still examining our next steps. 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."
Analysis - Roger Harrabin, Environment Analyst
In this latest skirmish in the war for clean air, the government looks to be holding a strong position in seeking to delay publication until June. First, ClientEarth have to decide if they can persuade a judge that public health issues are not covered by rules over election purdah.
Then a judge would need to be willing to make a ruling on the sensitive political issue of defining purdah. Even if the judge ruled against the government, ministers would be likely to appeal.
And that would spin out the process until after the election anyway. Campaigners see this as a calculated move by ministers wanting to dodge tough decisions on taxing dirty diesel cars.
They say a three-month delay leaves 10,000 more likely to die as air pollution continues. Others will note that the policy is already seven years delayed so it's worth another three months' wait to win the clean air war.
In the House of Commons on Monday, Environment Secretary Andrea Leadsom said the government had committed more than £2bn towards measures aimed at tackling air pollution, such as promoting the take-up of low emission vehicles.
She said the UK now had lower emissions of some key pollutants and added: "Due to the failure of Euro vehicle emission standards to deliver the expected improvements in air quality, the UK is among 17 European countries including France and Germany who are not yet meeting EU emissions targets for nitrogen dioxide in parts of our towns and cities.
"We are taking strong action to remedy that."
Labour MP Sue Hayman responded: "The situation has gone from bad to worse under this government's watch and has now escalated into what the Efra (Environment Food and Rural Affairs Committee) calls a 'public health emergency'."
She added: "Isn't it the case that the government is doing everything it can to avoid scrutiny because it is missing its own commitments, has no strategy and wants to yet again kick this issue into the long grass?"
While the deadline for publication passes at 16:00, it could take a few of days for the court to decide whether to grant the application.
ClientEarth chief executive James Thornton said: "Whichever party ends up in power after the 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."
A raft of recent studies and reports have linked air pollution to heart disease and lung problems, including asthma.
Last year, the Royal Colleges of Physicians and of Paediatrics and Child Health said outdoor air pollution was contributing to some 40,000 early deaths a year in the UK.
ClientEarth won a Supreme Court ruling against the government in April 2015.
That judgment ordered ministers to come up with a plan to bring down air pollution to within legal limits as soon as possible.
But ClientEarth was dissatisfied with those proposals, and took the government to the High Court in a judicial review. A judge ruled in favour of the environmental lawyers in November 2016.