The mayor of London has called on Prime Minister Theresa May to back Gatwick to expand aviation capacity in the south east.
Khan said he has written to May to urge her on making a decision on what should be a top priority for the government and agree with him that a second runway should be built at Gatwick Airport.
Newly-appointed transport minister Chris Grayling has said making a decision is a top priority, while chancellor Philip Hammond has said he would want to review the evidence.
Hammond previously said a decision should be taken when Dubai surpassed Heathrow as the world's busiest airport.
Khan said: “A final decision on airport expansion in the South East is long overdue and I urge you to make this a top priority of your Government. London’s economy needs additional aviation capacity in order to stay competitive with other major global cities and an early decision from you would show to the world that we are open for business.
Gatwick is the most viable option presented by the Airports Commission. A second runway at Gatwick will bring substantial economic benefits to London and the UK in terms of jobs, growth and investment, without the significant environmental cost to Londoners of expanding Heathrow.
I was elected on a manifesto commitment to oppose Heathrow expansion and whilst aviation expansion in the South East is of national importance the views of Londoners, who overwhelmingly oppose Heathrow expansion and who suffer the environmental consequences of it, must be taken into account.
London’s poor air quality is responsible for the premature deaths of thousands of people each year which is why I announced an ambitious package of measures to tackle this pressing problem and make it one of my top priorities as mayor. Heathrow is currently an air pollution hot spot and will be for years to come even without expansion, unless significant mitigating measures are put in place. Aviation expansion in the South East must not be at a further cost to Londoners’ health and well-being, and increased noise pollution is also a significant factor in my opposition to Heathrow expansion.
This is a vital decision for London and the UK and we must get it right. Therefore, I urge you to agree as swiftly as possible in favour of a second runway at Gatwick and rule out a third runway at Heathrow.
Khan has backed Gatwick over Heathrow for some time - though did flip flop on the issue last year.
Last month he unveiled a boost to investment at Gatwick of £200m with a visit to the aiport, bringin the total investment over the next five years to £1.2bn.
The news comes after it was announced in June that the airport expansion decision has been pushed back until "at least" October.
Business leaders have been pushing for a quick decision since the Davies Commission was published last July, recommended building a third runway at Heathrow.
The government said in December it would delay making a decision on airport expansion in the south east until at least this summer, saying "more work will be done on environmental impacts" in the interim.