Nick Clegg today ruled out supporting a third runway at Heathrow, saying that it would increase pollution and breach climate change targets.
Speaking on his weekly call-in radio programme on LBC, the deputy prime minister said he would not support any plan for expansion which damaged the environment.
"My point of view has always been that why I've always opposed the plans I've seen in the past for the expansion of Heathrow is because of the environmental effects," he told Nick Ferrari.
"I'm not going to endorse any plan of airport expansion which would increase the existing levels of noise and air pollution and would breach the climate change targets that we have all signed up to."
Lib Dem business Secretary Vince Cable also repeated his opposition to the proposals today, telling the Today programme that it is "highly unlikely" that Heathrow expansion will ever go ahead.
Clegg's comments will put pressure on the other two party leaders to make their views public on expansion at Heathrow.
David Cameron went into the last election with the promise that "no ifs, no buts, no third runway," but is now believed to be considering breaking this pledge after the next election.
Last week Ed Miliband denied reports that he is also wavering on his opposition to Heathrow expansion.
The Labour leader threatened to resign from the last Labour government over their policy of supporting expansion at Heathrow.
Earlier this week, the Airports Commission released a shortlist of options for airport expansion in the UK.
Two of the three options on the list were for expansion at Heathrow.
The report has caused widespread anger among Conservative figures in London.
Conservative MP has already threatened to force a by-election in his Richmond seat if Cameron accepts the proposals to expand Heathrow.
Yesterday Conservative London Assembly Member Tony Arbour also said he felt "betrayed" by the report and said that he would "withdraw his sword from its scabbard" on the issue.
He called on London Mayor Boris Johnson to lead a coalition of Conservative politicians opposed to expansion at Heathrow.