Gatwick Airport is due to unveil plans to use its emergency runway to increase capacity.
The airport's latest draft plan is set to be released on Thursday and will go out to public consultation, a spokesman said.
The emergency runway would be used for smaller aircraft departures, as part of future growth plans.
Opponents to expansion at the West Sussex airport have called it "a second runway by the back door".
An airport spokesman said it was "exploring how to make best use of its existing runways, including the possibility of bringing its existing standby runway into routine use".
"This would deliver an incremental increase in capacity that complements the expansion schemes of other airports across the South East."
The airport was also looking at using the existing runway "more intensely", aided by the use of modern technology, the airport spokesman told the BBC.
Campaign group Communities Against Gatwick Noise and Emissions said: "This is simply betrayal of communities of Sussex, Surrey and Kent who have already endured the increases in long haul movements this year by 24.1%.
"This is a second runway by the back door, how can communities ever trust Gatwick management again?"
Critics fear the proposals would mean up to a 30% increase in flights.
Peter Barclay, chairman of Gatwick Area Conservation Campaign, said: "We want to know what mitigation they propose to put in to counter the expected growth that this will produce because the impact in noise terms will probably be quite significant for those communities particularly to the north of the airport."
A legal agreement prevents the airport using both runways simultaneously, but that expires next year.
Jeff Alexander, chief executive of Gatwick Diamond Business, said the airport was a vital asset which should be allowed to " grow and prosper".
"It is a key driver of not only the regional but the national economy," he added.
"Goods and trade through the airport amounts to some £2.7bn, and overseas visitors through Gatwick contribute about £4.7bn annually, so it is a massive economic asset."
Gatwick's expansion plans were dealt a blow in 2016 when the government rejected its proposal for a new second runway, giving the go-ahead for Heathrow to build a third runway.
At the time, a Gatwick Airport spokesman said it was disappointed with the decision, which was "not the right answer for Britain".
In June, Gatwick Airport announced £1,11bn plans to expand part of the North Terminal, including six new departure gates.