The Airports Commission inquiry into airport expansion in south-east England may be delayed for further public consultation.
Sir Howard Davies, the Commission's chairman, has opted to examine more evidence on the problems caused by air pollution.
The consultation will begin immediately and end on May 29.
The delay comes after the Supreme Court last month ordered the UK to do more to improve air quality.
There are three remaining options for a new runway in the south-east — a second at Gatwick, a third at Heathrow, or extending one of the two existing runways at LHR.
The Commission had been expected to make its recommendation shortly after last week's general election.
In March, London mayor Boris Johnson blocked plans for a £200 million expansion at London City airport (see news, March 27).
And in January, Easyjet, which has its biggest base at Gatwick, threw its support behind London Heathrow's plan for a third runway (see news, January 30).
Tom Newcombe