Boris Johnson's Heathrow warning after Justine Greening's move
The Mayor of London, Boris Johnson, says the reshuffle shows the government wants to "ditch its promises and send yet more planes over central London".
Justine Greening, who is opposed to a third runway at Heathrow, has been replaced as transport secretary by Patrick McLoughlin.
"There can be only one reason to move her - and that is to expand Heathrow Airport," Mr Johnson said.
The idea was "mad" and he would fight it all the way, he added.
In a statement, the mayor said: "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. Even if a third runway was built, it would not do the job of meeting Britain's needs.
"If we are to remain Europe's premier business hub we need a new four-runway airport, preferably to the east of London, that addresses the problem of aviation capacity before it is too late, and business is driven into the arms of our European competitors.
"It is time for Patrick McLoughlin to look at all the options, including bolder solutions that would deliver massive benefits in jobs and growth.
"And it is time for the government to level with Londoners. Are they in favour of a third runway at Heathrow or not?"
Conservative MP Zac Goldsmith, an opponent of Heathrow expansion, tweeted that "Greening's appointment 11 months ago indicated the PM's position on Heathrow was solid. Yielding so easily suggests panic, not principle".
Friends of the Earth also criticised the replacement of Miss Greening, saying she had been "shunted out" and become a "victim of intense aviation lobbying over airport expansion".
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.