The Government was ordered to publish updated clean air plans by April 24, after the courts ruled existing plans to meet EU-mandated air quality limits, which are being broken across many areas of the country, were not sufficient.
But the Department for Environment, Food and Rural Affairs (Defra) has applied to delay publication of the report until after the election in June.
Reacting to the news, the Mayor of London said: "Toxic air quality is the biggest environmental health crisis of our generation and ministers have been appallingly slow to act on their duty to protect the public from this silent killer, which is estimated to cause around 40,000 early deaths every year across the country.
"I am deeply disappointed that ministers have missed a golden opportunity to show real leadership and urgently introduce new plans to deal with our filthy air.
“This could have been done well before the pre-election period started had the Government treated this as a priority as I do.
“Instead they have used the general and local elections as a smokescreen to hide their incompetence as they seek to extend the court-ordered deadline for publishing their draft plan.
"Everyone deserves the right to breathe clean air and any delay in addressing air quality is a slap in the face to every person struggling to breathe, from the young children in London growing up with under-developed lungs, to the millions of asthma sufferers in the UK."
Polluting cars could be forced to pay up to £24 a day to drive in central London from 2019 under plans previously unveiled by Mr Khan.
Theresa May has in the past indicated that she will not punish drivers of old diesel cars if the Government cracks down on the motors to protect the environment.
Drivers were encouraged to switch away from petrol under Tony Blair's government and Mrs May said that would be taken "into account" in future plans.
A Defra spokesman said: "We are firmly committed to improving the UK's air quality and cutting harmful emissions.
"We are seeking an extension to comply with pre-election propriety rules."
James Thornton, CEO of environmental lawyers ClientEarth, slammed the Government for its "very last minute" application.
He said: "We are urgently considering the Government's application to delay the publication of the draft Air Quality Plan which was received on Friday evening, less than one working day before the plans are due.
"It is far from acceptable that ministers have left this to the very last minute.
"The Government proposes to delay the publication of the Air Quality Plan despite the clear public health risk caused by illegal air quality.
"These plans are essential to safeguard public health and they should be put in place without delay.
"New plans were ordered by the High Court as a result of our case last year. This is not a political issue but a public health issue.
"Whichever party is in power, the British public need to see an air quality plan which relies on good scientific evidence and which ensures that people no longer have to breathe toxic air and suffer the grave consequences to their health as a result."