By Mohammed Abbas and Tim Castle
LONDON (Reuters) - British Prime Minister David Cameron tried to defuse a row over a potential expansion of London's over-stretched Heathrow airport on Wednesday, pledging to seek cross-party support for plans to boost British airport capacity.
Speculation has mounted that Cameron's ruling Conservative Party would go back on its pre-election promise not to build a third runaway at the airport after the transport minister who opposed expansion was replaced in a reshuffle on Tuesday.
Heathrow is Britain's biggest airport, the world's third busiest, and a hub for international flights, but its location near residential areas makes its expansion a toxic issue for local voters, green groups and the Conservatives' coalition partners the Liberal Democrats.
Business leaders say airport capacity must be increased to boost trade and routes to developing markets such as China.
The issue pits Britain's increasingly unpopular prime minister against well-liked London Mayor Boris Johnson, who opposes expansion at Heathrow and is tipped as a future Conservative Party leader, and risks straining the frayed coalition government.
The government is under pressure to speed up infrastructure projects as it seeks to revive Britain's finances, with its flagship austerity plan coming under attack as it fails to turn around the recession-hit economy.
"I will not be breaking my manifesto pledge," Cameron said during a stormy session of the weekly prime minister's question time, his first grilling after parliament's summer break. He said he would make an announcement on airport capacity in the coming days.
"What we need to do is build a process that hopefully has cross-party support, so we can look carefully at this issue and deliver changes that will address the problems of capacity that we will have in future years," Cameron said.
Cameron had agreed with the Lib Dems when they came to power in 2010 not to build a third Heathrow runway, but his intention to seek cross-party support to boost airport capacity opens the door to the plan after the current parliament ends in 2015.
Cameron's aides told reporters that the prime minister would contact the opposition Labour party before his announcement. Labour says it is "skeptical" about building a third runway.
A change in Conservative policy on Heathrow would come after a several U-turns since coming to power, most recently dropping plans to reform parliament's upper chamber, the House of Lords.
Johnson, riding a wave of popularity following London's Olympic Games, demanded Cameron rule out a third runway both during the current parliament and beyond, a call echoed by other Conservative politicians in areas under the airport's flight path.
"I think that is a profound mistake. It is not deliverable. It would do massive environmental damage," Johnson said.
"End this anxiety that's now building up that a U-turn is in progress and say no thanks, it's alright the policy as it has been, which is to say no to the third runway, both now and in the future, i.e. beyond the next 2-1/2 years," he said.
Conservative lawmaker Zac Goldsmith said he would not stand at the next election unless he was assured a third runway would not be built, calling on the government to "get off the fence".
"If we haven't heard by the summer next year what the government's view is in relation to airport capacity generally, I would personally read that as a hidden U-turn by the Conservative Party," he said.
After three consecutive quarters of economic contraction, Cameron and finance minister George Osborne are under pressure to find new ideas to boost growth while at the same time deflect calls for a fundamental re-think of their austerity program.
On Sunday Cameron promised to "cut through the dither" and ease planning and construction laws, but during a heated exchange in parliament opposition Labour leader Ed Miliband attacked Cameron's record.
"Over the last 2-1/2 years we have seen announcements on infrastructure - failed, announcements on housing - failed, announcements on planning - failed," he said.
Cameron responded by painting Miliband as subordinate to his finance spokesman Ed Balls.
"Apparently he (Miliband) still has to bring in the coffee every morning. That's just how assertive and butch the leader of the opposition really is," Cameron said.
(Editing by Louise Ireland)