A second county council has withdrawn its support for another runway at Gatwick airport because of the high level of local opposition.
West Sussex County Council (WSCC) voted by 37 to 26 to reverse its decision in July 2013 to support expansion of the airport.
Kent County Council withdrew its backing for a second runway in November, because of new flight paths.
Gatwick said it was disappointed by WSCC's decision.
The Airports Commission is considering three options for airport expansion in the South East - a second runway at Gatwick, a third runway at Heathrow or an extension to one of the existing Heathrow runways.
WSCC held a special meeting to consider its response to a public consultation that ends next month.
"The issue of a second runway at Gatwick is an extremely emotive subject," said Councillor Pieter Montyn.
"Councillors believed that Gatwick has not, to date, put forward sufficient evidence to counter the very serious environmental and noise fears of local residents.
"However, irrespective of our position, our role now is to start planning for the future.
"The decision about the runway will be made by government so we have a responsibility and duty to plan for either scenario - second runway at Gatwick or not."
The council's official response is contained in 20 pages of improvements it believes should be made before any proposal to expand Gatwick is submitted to the government.
The Airports Commission is expected to make its final recommendation later this year.
Gatwick said it believed expansion would bring a huge boost for the local economy, including thousands of jobs.
"We will continue to work closely with West Sussex County Council to illustrate the benefits that expansion will deliver," it said.
"Gatwick is doing as much as possible to reduce the impact of aircraft noise on local residents."