Research has suggested a new runway could be built at Heathrow without breaking pollution laws, it has been reported.
The BBC said the independent study, led by the University of Cambridge, measured poisonous nitrogen dioxide (NO2) levels using 40 sensors in and around the London airport.
London mayor Sadiq Khan made a last-ditch plea to Theresa May this week, calling for an extra runway to be built at Gatwick rather than Heathrow.
The shortlisted options being considered are a third runway at Heathrow, extending its existing northern runway, and a second runway at Gatwick.
Speaking about the Heathrow study, Professor 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."
He added: "By deploying a network of sensors we can tell directly from the measurements, what's been emitted locally from Heathrow airport and what's been blown in, mostly from central London. That's the real strength of the sensor network.
"The major result from this study is that we have tested the models far more critically than you can from a single measurement site."
In her keynote address to the Tory Party conference in Birmingham, Mrs May confirmed that the Government would "shortly announce" which project would get the go-ahead.