Pollution in London, Birmingham and Leeds will not reach safe levels until after 2030, more than 20 years later than the official European deadline, the government has been forced to admit.
Ministers had promised to make the air in British cities meet EU limits by 2025 at the latest.
However, new figures released by Defra this week show that the deadline for tackling them has been pushed back by up to another ten years.
The admission came as the government faced legal action in the European courts for their failure to meet the legal limits.
Client Earth, who brought the action, said the delay meant thousands more people across the UK would be made ill through dirty air.
"Another five years of delay means thousands more people will die or be made seriously ill," Client Earth lawyer Alan Andrews said.
"The UK needs to act now to get deadly diesel vehicles out of our towns and cities.”
Levels of nitrogen dioxide (NO2), linked to diesel engines have been in breach of legal levels since the European deadline passed in 2010.
The deaths of up to 4000 people each year in London alone are attributable to pollution levels.
High levels of pollution also exacerbate conditions such as asthma and can cause breathing difficulties in otherwise healthy people.
The Green party today called on the government to act to reduce the number of vehicles on the road.
"People's health has suffered directly because of the complacency and inaction of successive government ministers," Green London assembly member baroness Jenny Jones said.
"This latest admission about the extent of the air pollution problem reinforces the case against more road building in London and for the introduction of pay as you go driving to bring down fares and reduce traffic."
The government's admission comes as London's Oxford Street was named the most polluted street in the entire world, due to diesel emissions.
Levels of NO2 on the street were found to be up to ten times the safe and legal limit.
Defra today insisted pollution levels were coming down.
"We are investing heavily in measures to improve air quality and have committed billions to increase uptake of ultra-low-emission vehicles, sustainable travel and green transport initiatives," a Defra spokesperson said.
"As our understanding of NO2 evolves this must be reflected in our projections which is why we have revised these figures – work is under way to ensure compliance with EU limits in the shortest possible time."