Heathrow’s outgoing chief executive admitted the M4 would need to be diesel-free if a third runway was ever built at the airport.

Colin Matthews, who will leave his post as the airport’s boss in June, told aviation specialists Flightglobal the fleet of diesel engines travelling on the M4 would need to be replaced to fix the air quality at Heathrow.

It is the first time a senior Heathrow official has been so frank about the air pollution problems the airport is facing.

John Stewart, chairman of anti expansion group Hacan, said: “We commend Colin Matthews on his honesty but it is simply act of faith for the airport to believe that air pollution limits will be within the legal limits by 2026, the date a third runway would be expected to open, as the new runway would mean an extra 240,000 flights a year.”

The European Union has made clear that its air pollution legal limits set in 2010 must be complied with by 2020 or member states face hefty fines.

There are pockets around Heathrow which remain stubbornly above the legal limits caused by both the aircraft and the heavy traffic on the nearby roads and motorways.

A Heathrow spokesman said: “The most significant contributor to emissions around Heathrow is road vehicles.

“Pollution levels in central London are much higher than the boroughs around Heathrow.

“We believe we can add capacity at Heathrow without exceeding air pollution limits with a combination of new road vehicle emissions standards, new aircraft technology and increased use of public transport.”

The Airports Commission is currently assessing the case for a third runway at Heathrow and a second Gatwick runway and will release Government recommendations for future aviation capacity in summer 2015.