Gatwick is proposing to spend £500m to widen its emergency runway and bring it into daily use as a second runway, as part of a plan to bring more than 100,000 additional flights a year to the airport by 2032.

The core proposal in an ambitious 15-year masterplan published on Thursday shows that Gatwick, the UK’s second-biggest airport, in south-east England, hopes to serve up to 70 million passengers a year with two runways – almost as many passengers as Heathrow today.

Gatwick’s emergency runway could be widened by 12 metres to comply with safety requirements for a 210-metre centreline gap from the main runway, and be used for an additional 10-15 short-haul flights an hour to take off.

An expanded terminal, an additional aircraft pier of landing gates and work on roads around the airport could also be required to accommodate the extra passengers. The work would take two to three years to complete.

A 40-year agreement with local councils to remain a single-runway airport, using the standby only in emergencies or when the main runway is closed, comes to an end in 2019. Should Gatwick press ahead after public consultation to obtain planning permission in 2020, potentially the runway could be open by 2025, before Heathrow’s third runway.

Land will also be safeguarded for a possible third runway - the scheme Gatwick has long proposed to build, and which was shortlisted by the Airport Commission. Gatwick said it believed it remained a credible means of providing longer-term growth for the UK, but it would not currently pursue it while the government was backing Heathrow expansion, approved by parliament this summer.

Even without extra infrastructure, the airport expects to grow significantly, from 45 million to 60 million passengers per year, by flying more and larger planes on its existing runway.

Gatwick’s chief executive, Stewart Wingate, said: “As the UK heads towards an important new chapter, Gatwick’s growing global connections are needed more than ever, but this must be achieved in the most sustainable way. From using new technologies on our main runway, to the innovative proposal to bring our existing standby runway into routine use, our draft master plan offers agile, productive and low-impact ways of unlocking much-needed new capacity and increased resilience from within our existing infrastructure.”

The proposals will now go out for a 12-week public consultation. Local opposition groups fear increased noise, with the additional runway likely to concentrate more traffic over existing flight paths.

The airport said that the noise footprint would not expand, and that it could ban certain night flights.

But Sally Pavey, of Communities Against Gatwick Noise Emission, said: “You cannot have tens of thousands more flights without more noise. There will be no respite in the day.

“It’s outrageous that they can be announcing a second runway and safeguarding land for another one too. The infrastructure and roads around Gatwick are crumbling already due to the number of passengers – it does not have the surface access.”

But London politicians and business groups backed the scheme. The mayor, Sadiq Khan, said: “I’ve always been clear that the south-east needs additional airport capacity and that this could be delivered quicker and more cheaply at Gatwick, without the same impact a new runway at Heathrow would have.

“These new and innovative proposals from Gatwick in their masterplan could make a substantial contribution, creating the runway capacity so desperately needed in and around London.”

The CBI’s director-general, Carolyn Fairbairn, said: “London’s airports are set to be full in the next decade, so the CBI welcomes Gatwick’s highly productive proposals to deliver increased capacity that complements expansion schemes at other airports. This will drive trade and investment, create new jobs and help British businesses thrive.”

The masterplan’s publication comes after speculation that the airport’s biggest shareholder, the private equity firm Global Infrastructure Partners, would sell its 42% stake in the airport in the near future.