Gatwick airport has been accused of creating a second runway “by the back door” through plans to put its emergency runway into regular use, adding hundreds of extra flights every day.

The airport, the second biggest to serve London, lost out to Heathrow when the government’s Airports Commission recommended where the next full runway should be built in south-east England.

But Gatwick will publish plans on Thursday for an existing, parallel stretch of tarmac, currently used as a taxiway or main runway in emergencies, to be called into general operation – potentially increasing its capacity by 30%, or more than 80,000 extra flights a year.

The airport would not confirm details before publication and the start of a 12-week public consultation, but it is understood that the 2,500m second runway could be used for short-haul flights should the safety case be proved. It had been thought too close to the main runway to be used simultaneously.

A Gatwick airport spokesperson said: “Airports are required to publish new master plans every five years, setting out their future growth plans and Gatwick has consistently confirmed it will do so before the end of this year. The draft plan will set out for our local communities, partners, airlines and stakeholders three possible growth scenarios, which we will then open up for views and feedback.

“In line with recent government policy, Gatwick has previously set out it is exploring how to make best use of its existing runways, including the possibility of bringing its existing standby runway into routine use.”

Any such plans would ultimately need Civil Aviation Authority approval for safety, as well as air traffic control approval. However, unlike other airports, Gatwick does not have a legally binding cap on its number of flights. It handles more flights than any other single-runway airport.

The residents group Communities Against Gatwick Noise and Emissions (Cagne) called the plan “a second runway by the back door”. Spokesperson Sally Pavey said that expansion had been turned down by the Airports Commission and the airport did not have the transport links or facilities for more.

She said: “Extra flights mean thousands more passengers every day trying to get to Gatwick, but they are trying to increase capacity without any of the cost of the infrastructure.

“This is simply betrayal of the communities of Sussex, Surrey and Kent who have already endured 24% increases in long haul movements this year. This is a second runway by the back door. How can communities ever trust Gatwick management again?”

A 40-year agreement struck by Gatwick and local councils ruling out a second runway expires in summer 2019.

The Department for Transport said it would not comment without sight of the plans.

But the government’s National Policy Statement on aviation, which approved the building of Heathrow’s third runway, did encourage other airports to make best use of current capacity.

Gatwick’s latest plan could result in extra flights several years before Heathrow’s additional infrastructure, which is not expected to be operational until at least 2026.