A war of words has broken out between Heathrow and Gatwick about claims the west London airport’s expansion proposals are "unlawful".

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Fly away: Both airports want to expand, but who will win?

The Airports Commission re-opened its consultation process last month to hear views about air pollution if either of the two London airports were given the green light for expansion.

The consultation, from May 8 to May 29, asked for feedback on new evidence about air quality after a Supreme Court ruling said Britain needed to do more to tackle air pollution, having failed to meet EU limits.

But since the consultation closed, Gatwick has branded both Heathrow’s expansion plan options as unlawful and said if they were given the go ahead, they would cause further delay to the attainment of legally-binding limits.

At a hearing at the end of April, five judges ordered the Secretary of State for Environment to consult on strict new air pollution plans as a way for the Government to cut down the UK’s high nitrogen dioxide levels.

Stewart Wingate, chief executive officer at Gatwick, said air quality concerns had prevented Heathrow expansion in the past and air quality around west London was expected to exceed legal limits until beyond 2030.

He added: "The country cannot afford to make the wrong choice and risk years of delay on a project that is eventually halted because of its unacceptable impact.

"There is now an alternative - Gatwick has never breached legal air quality limits and would operate comfortably within these standards with a second runway.

"Gatwick’s expansion plans strike the right balance between delivering the extra airport capacity and taking the right steps to protect the environment."

Heathrow hit back and said it operated lawfully and the commission had been clear its north-west runway proposal could be delivered within legal air quality limits.

In a statement, a Heathrow spokesman said there were two locations adjacent to the M4 where EU air quality limits are not met at present, but added they were part of the "broader problem" London faces and are not directly linked to the airport.

The spokesman added: "Heathrow understands air quality is a real concern for local communities and an issue London needs to tackle urgently. We have a good track record of improving air quality, and have reduced emissions from airport activity by 16 per cent over five years."

The commission is expected to publish its report about airport expansion this summer.