The Liberal Democrat leadership has been defeated over planned changes to the party's opposition to airport expansion in south east England.
Members voted for the current policy which also specifies "no net increase in runways across the UK as a whole".
Party leader Nick Clegg had supported amending the policy to allow Gatwick Airport to be expanded.
But this option was comfortably rejected by members at the Liberal Democrat conference in Glasgow.
BBC assistant political editor Norman Smith said the vote was a "significant blow" to Mr Clegg, presenting a dilemma in the event of coalition talks following the next election because he would be bound to reject all likely expansion options.
However although party policy, it will not necessarily feature in the next Lib Dem manifesto.
Speaking after the vote, Cambridge MP Julian Huppert said the party "has made a clear decision", adding the issue would be "an important part of coalition discussions" with other parties if there was another hung Parliament.
Supporters of the change, including Mr Clegg, had argued airport expansion could take place without increasing carbon emissions, thanks to technological advances.
The government set up a commission, chaired by Sir Howard Davies, to consider ways of expanding airport capacity. The commission, which will publish its final report next summer, has shortlisted three options: a third runway at Heathrow, lengthening an existing runway at Heathrow and a second runway at Gatwick.
The airport expansion amendment was tabled by Solihull MP Lorely Burt and Stephen Gilbert, who represents St Austell and Newquay.
Ms Burt told activists that rejecting another runway would not reduce air travel, but instead benefit the UK's competitors, such as Paris and Amsterdam, at the expense of the UK.
Speaking on BBC Radio 4's Today programme, Lib Dem Energy Secretary Ed Davey said he would back the amendment to exempt Gatwick.
He said: "What we're saying is the environmental criteria we have always had may well be able to allow expansion elsewhere given technological change.
"It's not a U-turn on environmental criteria - that's the key point."
He said: "We're not against flying, we're not against people using their cars, we're not against people actually enjoying life and the economy growing.
"We just want to do that in a low-carbon way, we've always said that."
The Lib Dems' "pre manifesto" for next year's general election, published last month, said: "We remain opposed to any expansion of Heathrow, Stansted or Gatwick and any new airport in the Thames Estuary, because of local issues of air and noise pollution".
The party's last general election manifesto promised to "cancel plans for a third runway at Heathrow and other airport expansion in the South East".
Business Secretary Vince Cable has said expansion at Gatwick was "a preferable alternative" and "less problematic" than expansion at Heathrow, which is near to his Twickenham constituency.
Roger Harrabin, environment analyst
Environmentalists do not believe that the case for airport expansion has been made at all - they doubt UK plc will lose Chinese business if travellers have to wait an hour for a connection at Schiphol or Frankfurt.
Mr Davey said that zero carbon flight is imaginable. Indeed there is already a solar plane - but it has wings the size of a jumbo and carries just the pilot.
Conventional engines are becoming more efficient - but future improvements are projected to save at most 35% of emissions per flight.
Other "green" options like biofuel are controversial because they involve cutting down tropical forests.
But Peter Chivall, of the Green Liberal Democrats group, urged the party leadership to "see sense".
He said: "We know that the only way you can restrict aviation and restrict the amount of greenhouse gases emitted by planes is to restrict the number of runways. And that's the way we have to go."
Speaking before the vote, health minister Norman Lamb said he was "really torn" on the issue.
Mr Lamb said it was important to weight "the imperative of tackling climate change" against the need to maintain trade links with the developing world.
Lib Dem Transport Minister Baroness Kramer, who until 2010 was MP for a seat under the Heathrow flightpath, said the government must be prepared to defy the recommendation of Sir Howard.
"Any government that says 'we will automatically do what Davies recommends' is abdicating the responsibility they were elected to exercise," she told a fringe meeting.