Boris Johnson has insisted he would not allow a third runway to be built at Heathrow Airport, amid speculation that senior Tories want to revisit the idea.
The Conservative Mayor of London, fighting for re-election in May, said the move would be an "environmental disaster" and "intolerable" for people in the west of the capital.
The Tories ruled out the prospect of a third runway in their 2010 manifesto, but both Prime Minister David Cameron and Chancellor George Osborne last week acknowledged the need for airport expansion in the South East.
Mr Cameron said the Government was prepared to consider the possibility of a new airport in the Thames Estuary, an idea that has been advanced by Mr Johnson and dubbed the "Boris Island".
But there were reports that Mr Cameron and Mr Osborne were prepared to look again at options for Heathrow, in a move that would anger their Liberal Democrat coalition partners and Tory MPs in west London.
One possibility was said to be to develop RAF Northolt, which is about six miles from Heathrow, into an effective third runway.
Tory sources insisted that their commitment not to build a third runway remained. Transport Secretary Justine Greening is to set out plans on aviation in the summer.
Mr Johnson, who faces serious damage at the polls if he becomes associated with plans for a third runway, said Heathrow could not be "endlessly" expanded.
"A third runway would be an environmental disaster. It would mean a huge increase in planes over London, and intolerable traffic and fumes in the west of the city - and it will not be built as long as I am Mayor of London," he said.
"That is why the Government is right to look at all new solutions for extra aviation capacity except the third runway at Heathrow. I look forward to engaging with Justine Greening's consultation this summer."