Gatwick has hit back at research suggesting that rival Heathrow would not breach emissions limits if it won the right to expand.

The West Sussex airport claimed to have new evidence that shows 19 of 26 separate monitoring sites around Heathrow are up to 162% of the legal NO2 limit.

The data – collected over three months along local roads and residential areas – is more comprehensive than monitoring stations run by local authorities, which provides only a ‘snapshot’ of air quality in the Heathrow area across a limited number of areas, Gatwick suggested.

Gatwick published the data to demonstrate how unlikely it is that Heathrow’s air quality will improve in time, and by enough, to accommodate the large amount of pollution that construction traffic will bring to the area from 2021 – the date construction is planned to start.

The impact of construction related traffic alone is likely to ensure that air quality around Heathrow is over legal limits – making expansion there unlawful, according to Gatwick ahead of an imminent decision by the government on expanding airport capacity in the south-east.

Construction-related traffic includes 223,000 delayed M25 journeys each day for up to seven years while the motorway is put into four tunnels.

Gatwick chief executive Stewart Wingate said: “Air quality around Heathrow continues to soar way above legal limits today and it is inconceivable that it could improve fast enough to make expansion legal anytime soon.

“Gatwick has never breached legal air quality limits. We would have a new runway operational by 2025 without any of the environmental, construction and financial delays that Heathrow will inevitably face.”