The independent research measured poisonous nitrogen dioxide levels, using 40 sensors in and around the airport, according to BBC News, which has seen the report.

The research was used to predict what would happen in the future. However the Aviation Environment Federation (AEF) warned that the study was “highly speculative”, and that it offered no guarantee pollution levels would fall.

The research was lead by the University of Cambridge, with no formal links to any airport or government.

Prof Rod Jones from the University of Cambridge told the BBC: "If there is the development of a third runway, we expect there to be a marginal increase in NO2 coming from the airport itself, but that would be against the background of reduced NO2 from other traffic, because of Euro 6 engines and electrification of the traffic fleet."

It means any increase would likely come from traffic on the roads, rather than planes in the air, because its the cars where the bulk of the poisonous nitrogen dioxide gases come from.

An announcement on airport expansion is due later this year - earlier this week transport secretary Chris Grayling insisted a decision on a new runway for the south-east was “right at the top of my in-tray”.