London Gatwick's operator has put forward a formal proposal for a parallel second runway, to the south of the airport, which could open in 2025.
Its proposal, submitted to the UK's Airports Commission, will cost some £5-9 billion ($7.5-13.6 billion) which, it claims, would be a "fraction" of the expense of expanding London Heathrow.
Gatwick's operator says it does not have a "preferred" second runway option but is focusing on a parallel strip to the south - the precise operation of which would depend on the separation distance.
It says it has "decided to discontinue" studies into a northern runway, because the benefits "appear insufficient" given the scale of the landscape impact.
The operator also believes that a southern runway, staggered to the east, would present "very significant" complexity and cost problems, particularly relating to construction over a main rail route.
If a second runway is built as a closely-spaced parallel - with less than 760m (2,500ft) separation - then it could not function independently of the current one. But it could nevertheless support 67-70 movements per hour.
Two other options would space the parallels more widely, enabling them to operate independently and giving greater flexibility to handle arriving and departing traffic. Spacing them more than 760m apart would support 75 hourly movements while a 1,035m separation would increase this figure to 95-100.
These various options would allow the airport to handle 60-87 million passengers annually by 2050.
"Our evidence shows clearly that an additional runway at Gatwick would best serve the needs of all passengers, and give certainty to airlines, communities and businesses," says Gatwick chief executive Stewart Wingate. The airport would form part of a "constellation" of three major airports for the capital, he adds, and could be privately financed without "substantial" state subsidy.