The UK Government is facing the threat of fresh legal action after it sought to delay publication of its plan to tackle air pollution until after the June 8 General Election.

Ministers had been given until 4pm on Monday by the courts to set out draft measures to tackle illegal levels of nitrogen dioxide pollution, largely caused by diesel emissions.

It followed a court ruling last year that existing measures put forward by Theresa May’s administration did not meet the requirements of the law.

But in a surprise move late on Friday, the Department for Environment, Food and Rural Affairs lodged a fresh application with the High Court to postpone publication until after June 8.

It argued to move was necessary in order to comply with election "purdah" rules on government announcements during the election period.

The environmental law firm which brought the original case against the Government, ClientEarth, said it was 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 James Thornton, ClientEarth’s Chief Executive.

"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.

"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.

"The Government has had five months to draft this plan and it should be published," he added.

Although the deadline for publication passes at 4pm, it is thought that it might be a couple of days before the court decides whether or not to grant the application.

Sue Hayman, the Shadow Environment Secretary, said: "With nearly 40 million people in the UK living in areas with illegal levels of air pollution, it is simply not acceptable for ministers to hide behind the General Election to delay publishing plans to improve air quality.

"Purdah rules exist to stop one party using the machinery of government for their electoral advantage, not to be used as an excuse to delay acting on vital public health matters. We trust that the court will recognise this.

"While the Conservatives shirk responsibility, Labour will deal with the dirty air damaging the lives of millions of British people," she said.

Ms Hayman made clear a Labour government would produce an air quality plan within 30 days of taking office and legislate for a new Clean Air Act.

Asked about the Government’s reasoning, the Prime Minister’s spokesman said: “Following advice from the Cabinet Office on April 13, we sought a short extension to the publication of the draft plans in order to comply with local election proprietary rules and then, subsequently, in light of the General Election on Friday April 21 we sought an application to consult on the draft air quality plan shortly after the General Election and to publish the plans by September 15.”

After MPs voted to allow for a snap election last Wednesday, guidance was issued to the UK’s 400,000 civil servants the following day about purdah rules, which came into effect at midnight on Friday.

During purdah ministers continue to run the country but must defer from making any major policy decisions, signing any large or contentious procurement contracts or making any other significant long-term commitments until after the election; unless, that is, postponement would be detrimental to the national interest or wasteful of public money.

In 2015, purdah began when parliament was dissolved, which means that ahead of the 2017 General Election the period of restriction is starting almost two weeks earlier than it did two years ago. The dissolution of parliament will take place on Wednesday May 3.