Labour has accused the government of failing to act on air pollution, which it says is a “public health emergency”.
The party said millions of people in the UK were living in areas with illegal levels of nitrogen dioxide (NO2) and called air pollution levels a “national scandal”.
It has vowed to bring in a new Clean Air Act, including a network of “clean air zones” if it is elected.
The government says it is committed to improving the UK’s air quality.
Jeremy Corbyn said there were “terrible levels” of air pollution in certain parts of the country which had to be “dealt with”.
The Labour leader suggested there needed to be a policy on the “phasing-out of diesel” but it would have to be done in a way that ensured “you don’t punish people”.
It comes after ministers sought to delay the release of a key report on its plans to meet EU air pollution standards – regularly broken in many parts of the UK – which it had been ordered to publish by Monday.
The Department for Environment, Food and Rural Affairs went to court on Friday seeking leave to delay it until after the 8 June general election, saying it was necessary “to comply with pre-election propriety rules”.
Shadow environment secretary Sue Hayman accused the government of “attempting to use the general election as cover for their failure to act on air quality”.
She said it was “unacceptable” for the government to apply for the delay “less than one working day before it is due”.
Ms Hayman continued: “The UK air pollution crisis under Theresa May’s government is a national scandal and public health emergency.”
She told the Guardian that Labour would not allow the Conservatives to use the general election or Brexit to “kick this issue into the long grass or water down standards”.
Labour says almost 40 million people are living in areas with illegal levels of NO2.
This figure was based on data released in 2015 which identified 169 local authorities as exceeding the limit.
Outdoor air pollution contributes to some 40,000 early deaths a year in the UK, say the Royal Colleges of Physicians and of Paediatrics and Child Health.
NO2 comes from sources including factories and vehicles, particularly diesel engines.
The government was told to take immediate action by the High Court in April 2015 to cut air pollution after it was found the UK had breached EU limits for NO2 in the air.
Campaigners ClientEarth, an environmental law firm, took the case back to court last November saying the government’s plans did not go far enough.
Judges ruled in their favour saying the government’s proposals to improve air quality were “woefully inadequate” and ordered it to publish an updated draft plan by Monday.
The UK was sent a “final warning” over air pollution by the European Commission in February.
It said limits had been repeatedly exceeded in 16 areas including London, Birmingham, Leeds, and Glasgow.