Environmental law firm ClientEarth has warned the UK Government it could be taken to court over continuing levels of air pollution
In April the Supreme Court ruled an immediate plan was needed after the UK breached EU limits for nitrogen dioxide (NO2).
The government said it was committed to cleaning the air and had delivered its plans accordingly.
However ClientEarth, which took the original case, said the plans still do not protect health.
It says the government needs to improve policies to lower pollution levels
The firm has given ministers 10 days to respond.
It’s estimated 40,000 people are estimated to die prematurely every year in the UK because of bad air quality.
A significant contributory factor is due to nitrogen pollution from diesel vehicles.
Ministers have responded by creating special anti-pollution zones in Leeds, Southampton, Birmingham, Nottingham, Derby and London.
They say they have also committed £2bn from 2011 to improving standards of buses, dustbin lorries and fire engines.
But they still do not envisage that the air will meet EU health standards around the UK until 2020 - and 2025 in London because of the delay in getting older dirtier vehicles off the roads altogether.
Alan Andrews from ClientEarth told the BBC the government had itself to blame for failing to act sooner against diesel cars.
“Throughout our five-year legal battle the government have claimed they couldn’t achieve legal limits because of the problems with the EU standards for diesel vehicles not delivering pollution reductions under normal driving conditions - but they failed to investigate why.
“Then they lobbied the EU to water down new regulations which will require new diesel cars to meet emissions limits on the road. As a consequence, new diesel cars will be able to emit double the emission limit until 2021.”