London mayor Boris Johnson has attacked the demotion of Justine Greening in the Cabinet reshuffle - claiming it shows the government is intent on the "simply mad" expansion of Heathrow Airport.
The influential Conservative said he would fight any moves to build a new runway at the busy London terminal "all the way" and accused ministers of failing to be straight with Londoners.
Greening, whose Putney constituency lies under Heathrow flightpaths, has been a vociferous opponent of a third runway.
And that stance appeared to lie behind prime minister David Cameron's decision to shift her to the international development brief and replace her with ex-chief whip Patrick McLoughlin.
Downing Street insists the government's position is unchanged since the coalition agreement of 2010, which committed ministers to scrapping extra runway plans drawn up under Labour.
But it has launched a consultation process on future airport capacity and chancellor George Osborne sparked speculation over a possible U-turn by saying he believed more runway capacity was needed in the south-east of England and all options should be examined.
Johnson, widely seen as a challenger to Cameron to be the next Conservative leader, said Greening has been "a first-rate transport secretary".
"There can be only one reason to move her - and that is to expand Heathrow airport," he said in an angry statement following the reshuffle.
"It is simply mad to build a new runway in the middle of west London. Nearly a third of the victims of aircraft noise in the whole of Europe live in the vicinity of Heathrow.
"Now it is clear that the government wants to ditch its promises and send yet more planes over central London.
"The third runway would mean more traffic, more noise, more pollution - and a serious reduction in the quality of life for hundreds of thousands of people.
"We will fight this all the way."
He said that a new four-runway airport should be built instead, preferably to the east of London, to address the growing need to expand the UK's aviation capacity.
"It is time for Patrick McLoughlin to look at all the options, including bolder solutions that would deliver massive benefits in jobs and growth," he said.
"And it is time for the government to level with Londoners: are they in favour of a third runway at Heathrow or not?"
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Friends of the Earth executive director Andy Atkins said: "Justine Greening is a victim of intense aviation lobbying over airport expansion and an economic argument that simply doesn't add up.
"She's been shunted out of her transport job because of her opposition to a third runway at Heathrow.
"We don't need more airports or runways in the South East - they would have a devastating impact on local communities and the environment and undermine UK action on climate change."
The switch of Greening was attacked as a "panic" measure by Tory MP and environmental campaigner Zac Goldsmith.
Goldsmith has already said he will not be a Tory candidate at the next general election if the party does not explicitly oppose a third runway for Heathrow.
Responding to the reshuffle on Twitter, he said: "Greening's appointment 11 months ago indicated the PM's position on Heathrow was solid. Yielding so easily suggests panic, not principle.
"Real leadership requires clarity, not subterfuge. It's time for the Govt to be honest: has it changed its view on Heathrow; yes or no?"