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.
Ms 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.
Mr Johnson, widely seen as a challenger to Mr Cameron to be the next Conservative leader, said Ms 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."