Nearly half of London's roads will still be over legal pollution limits in 2020, according to the Green Party.
London Green Party leader Jenny Jones said 45% of the main road network will have illegal and harmful levels of nitrogen dioxide (NO2) by 2020.
She said the predicted pollution levels used modelling techniques set out in the mayor's 2010 Air Quality Strategy.
But the mayor's office said NO2 levels had decreased by 20% since Boris Johnson took office in 2008.'London-wide measures'
Under European Union air quality laws, daily pollution levels must not be above the legal limit on more than 35 days in a calendar year.
Big fines can be levied if cities are found to have breached EU law.
Ms Jones said: "Boris Johnson and the government's dithering on tackling air pollution meant we missed meeting legal limits on the harmful to human health emission nitrogen dioxide in 2010, and will still be over the limits a decade later."
A spokeswoman for Mr Johnson said: "Since the mayor took office emissions of oxides of nitrogen are down by an estimated 20%.
"That is because of an ambitious package of London-wide measures including building Europe's largest fleet of low emission hybrid buses, retiring the oldest taxis and introducing tighter emission standards for lorries and vans.
"Clearly there is still more to do, which is why the mayor has just allocated the first £5.4m of his new £20m Air Quality Fund to help boroughs tackle local air pollution."