Gatwick says it has abandoned plans to build an additional runway but will bring the airport's emergency runway into "routine use" by 2020.
The airport said it is preparing a planning application to get the second airstrip ready so it can be used alongside the main runway for departures.
If the plans are approved, the airport would aim to be serving around 70million passengers by the early 2030s.
The airport currently caters for around 46m passengers, meaning the capacity would be increased by around 50 per cent.
Plans for an additional runway, which it has abandoned as part of the masterplan, would have served up to around 92m passengers.
Last year Heathrow – which is the location chosen by the government for a new full-length runway – handled 78 million passengers.
The airport, the second largest in the UK after Heathrow, said it hoped the runway would be in use by the middle of 2020.
The announcement follows public consultation, in which two-thirds of people said the airport should make the most of its existing runways.
Another public consultation will be held once the planning application for the standby runway is submitted.
Gatwick said the plan is the "most sustainable way" for the airport to expand over the next 15 years.
London Gatwick CEO Stewart Wingate said: “The plans would deliver additional capacity for Gatwick, which will provide choices for the future – including incrementally growing our airport to meet demand and continuing to provide solid operational performance for passengers and airlines.
"This would be the biggest private investment for the region in the coming years, which would result in significant local economic benefits, including new jobs for the area.
“Gatwick’s global connections are needed more than ever but as we take our plans forward, we must do so in the most sustainable and responsible way and in full partnership with our local councils, communities, passengers and partners.”
The announcement has already drawn criticism from campaign groups with Communities Against Gatwick Noise and Emissions saying: "This is simply a second runway by stealth."
They added: "To use the emergency runway alongside the main runway is in affect a second runway as it will have to be moved by some 12 metres to allow it to be used.
"As such it is a second runway without the full parliamentary scrutiny or any funding for our roads or railway line that will see a huge increase in passenger and workers numbers migrating into Gatwick."
Gatwick currently consists of the main runway and the standby runway, which is used certain circumstances, including when there is maintenance on the main runway.
The airport began campaigning for a new runway when management published proposals in 2013 as a solution to the chronic shortage of aviation capacity in South East England.
Its expansion plans were dealt a blow in October 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".
It disputed the Airports Commission's findings that the economic benefits of a Heathrow expansion would be greater.
Gatwick is the second-busiest airport in the UK and the busiest single-runway in the world in terms of flight movement.
It serves more than 230 destinations in 74 countries for 46 million passengers a year as well as generating around 85,000 jobs nationally, with 24,000 of these located on the airport.