Air pollution on roads around Heathrow will continue to breach EU limits in 2030 even without a third runway, new figures suggest.
Air quality on the A4 beside the airport will be the third filthiest in the UK in 15 years time, behind only two spots on the A501 at Marylebone, according to latest government forecasts.
Anti-expansion campaigners claim the statistics, published by the Department for Environment, Food and Rural Affairs, could be a 'show-stopper' for hopes of a third runway at Heathrow.
The figures suggest average nitrogen dioxide levels at two points on the A4 north of Heathrow will be 48 and 47 micrograms per cubic metre in 2030 - comfortably in excess of the EU threshold of 40.
The A501 at Marylebone is predicted to remain the country's worst offender, despite NO2 levels there plummeting from 132mg/m3 in 2013 to 49 in 15 years time.
The A4 and A501 are forecast to account for seven of the UK's 10 most polluted spots by 2030, including the Hammersmith flyover.
The statistics, published in response to a Freedom of Information request by the campaign group Clean Air in London, were seized upon by the anti-Heathrow expansion pressure group HACAN.
Its chairman John Stewart said: "These dramatic figures suggest once again that air pollution could be a show-stopper as far as a third runway in concerned."
Annual flights at Heathrow would rocket by more than 50 per cent to 740,000 with a third runway, meaning a big increase in the number of passengers travelling to the airport.
Bosses at Heathrow have proposed new rail and bus links should a new landing strip get the go-ahead and are also considering introducing a congestion charge for passengers coming by car.
A Heathrow spokesman said expansion would only go ahead if strict environmental limits were met.
"Heathrow is committed to playing its part in meeting local air quality limits by incentivising cleaner aircraft and airside vehicles and increasing public transport use by both passengers and employees," he added.
The UK and other member states face fines if they continue to exceed EU air pollution limits in 2020.
The Airports Commission, which is considering rival bids for expansion at Heathrow and Gatwick, is expected to make its final recommendations in June.