Gatwick Airport has revealed plans to widen its emergency runway to bring it into full passenger use.
The scheme would enable the UK's second busiest airport to accommodate up to 109,000 extra flights a year.
Opponents of Gatwick expansion claim the proposal would increase the problems of noise, air pollution and road congestion already suffered by surrounding communities.
Gatwick's bid to build a new second runway was rejected by the UK Government in favour of Heathrow expansion in December 2016.
It says it could begin using its emergency runway for regular flights by the summer of 2025, just months before Heathrow's third runway is due to open.
The centre lines of Gatwick's main and emergency runways are separated by 198 metres.
The plan involves increasing the gap by widening the emergency runway by 12 metres.
The main runway would continue to be used for larger planes to take off and all aircraft to land but smaller aircraft such as the A321, A320 and B737 could take off from the emergency runway.
The airport expects to spend up to #500 million on the airfield works. Once opened, annual flight numbers are expected to increase from 281,000 today to as many as 390,000 in 2032-33, according to Gatwick's figures.
The proposal is part of a draft master plan, which also includes using new technology to increase capacity on the main runway and safeguarding the land where a new runway would be built if permission is obtained in the future.
A 40-year-old planning agreement with West Sussex County Council stating the standby runway can only be used in an emergency or when the main one is closed for maintenance expires next year.
Gatwick bosses say the conversion of the runway to full use would meet all international safety requirements and not increase the airport's noise footprint.
A detailed planning proposal and full public consultation would be required before it could go ahead.
The airport's chief executive Stewart Wingate said its master plan is an "innovative proposal" that offers "agile, productive and low-impact ways of unlocking much-needed new capacity".
Peter Barclay, chairman of the Gatwick Area Conservation Campaign, accused the airport of "attempting to get a second runway via the back door" and pledged to "fight this proposal tooth and nail".