The UK Government has been accused of "pure political expediency" in 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 courts ruled existing plans to meet EU-mandated air quality limits were not sufficient.

But in a surprise move on Friday night, the Department for Environment, Food and Rural Affairs[Defra] lodged an application with the High Court to postpone publication until after the June 8 poll.

Tony Lewis, head of policy at Chartered Institute of Environmental Health, led criticism of the move, saying: "They've had months to get their air quality plans ready and using purdah as an excuse is pure political expediency."

However, in an emergency statement Andrea Leadsom, the Environment Secretary, told MPs that it was "not appropriate" to publish the plan during the pre-election period and pledged to unveil the draft proposals on June 30.

Responding to an urgent question from Labour in the Commons, she said the move would not delay the implementation of the plans and insisted ministers were deeply committed to improving air quality.

"In accordance with the guidance covering both the local and general elections, the proprietary and ethics team in Cabinet Office have told us it would not be appropriate to launch the consultation and publish the air quality plan during this time.

“I also want to make very clear that we now have entered a period of time where we are strongly advised not to be publishing consultations and so what we are trying to do is a very short extension, which we do not believe will make a difference to the implementation of our plans but at the same time we are safeguarding our democracy."

But Sue Hayman, Ms Leadsom’s Labour shadow said it was unacceptable to hide behind the election and accused ministers of presiding over a public health emergency.

"Only a Labour government will legislate for a new Clean Air Act setting out how we would tackle air pollution that damages the lives of millions while this Conservative Government continues to shamefully shirk its legal responsibilities and puts the health of millions at risk."

There has been speculation the clean air plan - which would be published in full on September 15 - 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.

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.

Within months, the UK Government is facing the prospect of being hauled before the European Court of Justice unless it shows it is dealing with nitrogen dioxide pollution in 16 areas of the UK.

Glasgow has been described as "the most polluted city in Scotland" by environmental lawyers ClientEarth over what it called "illegal and harmful levels" of nitrogen dioxide[NO2], which comes mostly from diesel vehicles.

New EU data shows Glasgow has the highest average annual concentration of NO2 outside of London, with 68 micrograms of NO2 per cubic metre of air; 70 per cent higher than the Euro limit.

A recent survey revealed Scotland's most polluted street was Hope Street in Glasgow, with St John's Road, Edinburgh, second. Similar high emissions are to be found in streets in Aberdeen, Dundee, Falkirk, Perth, Crieff and Cambuslang, South Lanarkshire.