Campaigners suggest a high speed rail-link instead of an extra runway
A growing number of Labour MPs have urged ministers to rethink their plans for a third runway at Heathrow airport.
MPs from all parties have signed a Commons motion urging the government to look at "alternative solutions" to take into account noise and air pollution.
The Tories and Lib Dems oppose a third runway but 28 Labour MPs are among the 78 who have signed the motion.
Gordon Brown told MPs he backed a third runway "in principle" but was looking at all "environmental considerations".
A decision on the runway is expected before the end of the year.
New Transport Secretary Geoff Hoon said he would be making a decision after he had studied a summary of the 70,000 responses to the consultation.
He told the transport select committee it was important to get it "right" because "some communities will potentially be paying a heavy price for something that is in the wider interests of the country".
He denied Labour MPs campaigning against the runway were an "irritant" to the government, saying they were entitled to express their views.
In its 2003 aviation White Paper, the government said it supported Heathrow's development subject to strict air quality and noise level targets being met and as long as public transport access could be improved.
Operator BAA has said Heathrow is "jam-packed" and needs a third runway to remain competitive globally.
But environmentalists said the proposed expansion would have a serious impact on hundreds of thousands of homes in the area.
The motion, tabled by backbench Labour MP John Grogan, has been backed by a host of MPs in constituencies close to Heathrow but has also received support from MPs as far afield as Cornwall and North East Scotland.
It says the government has been too reliant on information supplied by BAA in assessing the case for expansion and a consultation document on adding capacity at the airport was "deeply flawed".
It urges ministers to consult on the opportunities for high-speed rail links between Heathrow and major UK cities as well as the scope for other airports around the UK to handle more long-haul flights.
Newspaper reports have suggested several members of the cabinet are increasingly sceptical about the merits of a third runway.
Mr Grogan said the decision was "not set in stone".
"The government has said that it is minded to approve a third runway but only if it can satisfy local environmental concerns...and I don't think they're going to be able to do that," he told the BBC.
"And I think it would be very unwise for the government to give the green light and then almost certainly face legal action next year in the long run-up to the general election."
Shadow transport secretary Theresa Villiers said the motion "raises the pressure on the government and on Labour MPs" and was part of a "broad coalition of opinion against a third runway".
At prime minister's questions earlier, Conservative MP Mark Prisk suggested some ministers were "lobbying against" a third runway and asked why they were still part of the government.
Mr Brown replied: "We said as a government that we supported in principle a third runway, after all there are five runways in Amsterdam, five runways in Paris, four in Frankfurt and we are talking about a third runway at Heathrow.
"But we said we would look at all the environmental considerations and that is what we are doing at the moment and we will come back to the house in due course."
The 2M Group, made up of London councils and environmental groups, has said a £20bn high speed cross country rail link with connections to Europe could replace the need to expand Heathrow.
This plan is backed by the Conservatives, who have promised to scrap the airport's expansion and invest billions in new rail lines.
The Lib Dems also oppose the third runway and back an extended high-speed rail network, taking in Heathrow.
But a group of high-profile businesses, backed by the CBI and British Chambers of Commerce, have insisted the runway must go ahead "within strict environmental standards".