The decision on a possible third runway for Heathrow will be made next month
Consultation over the need for a third runway at Heathrow has been a "complete sham", the Conservatives have claimed.
In a Commons debate on Heathrow expansion, shadow spokesman Theresa Villiers said the government had already made up its mind to go ahead.
But Transport Secretary Geoff Hoon said any approval would be subject to "stringent" environmental criteria.
Airport owner BAA and its main customer British Airways say the runway is vital if the airport is to stay competitive.
The Conservatives and Lib Dems oppose the idea of a new runway, as do environmental groups, while some backbench Labour MPs are calling for a rethink.
Prime Minister Gordon Brown has said a final decision, due next month, would be taken after "full consideration" of the environmental implications highlighted in the recent public consultation.
In the Commons, Mr Hoon said it was right that a decision was taken soon to "bring this period of uncertainty to an end" for those living close to the airport and for the country as a whole.
He denied the decision had already been made, saying ministers backed the need for additional runway capacity in south east England in principle - but would only approve the project if strict guarantees on air quality and noise levels could be satisfied.
He acknowledged the issue "aroused strong feelings" but argued that Heathrow was already operating at full capacity.
If nothing was done to remedy this, he added, flights and jobs would be lost to airports on the continent, damaging the economy.
He accused the Tories of taking "easy, populist decisions" on the issue and challenged those who wanted to "sit on their hands" to explain what the alternatives to more airport capacity were.
For the Tories, Theresa Villiers said the government's "mind has been made up for a long time" on a third runway.
She said the environmental costs outweighed the economic benefits of expansion and claimed the government "was deaf to the concerns of people about the environment".
There was a strong case for new high-speed rail lines as a "realistic and viable" alternative to the "thousands of flights clogging up Heathrow", she added.
More than 140 MPs - including 50 Labour MPs - have signed a Commons motion urging the government to consider the alternatives to a new runway.
John McDonnell, Labour MP for Hayes and Harlington - the constituency in which the runway will be built - said thousands of residents would lose their homes if it went ahead while thousands more would face "unliveable" conditions due to noise and pollution levels.
Assurances from successive governments about the limits of the airport's expansion would mean nothing if the runway went ahead.
"There has been a litany of lies and deceit over the development of the airport," he said.
But former transport minister Tom Harris said it was "fantasy politics" to argue that expansion could be put on hold indefinitely.
"Without a third runway, Heathrow will be left to wither on the vine and it will cease to become the economic powerhouse for our country that is currently is," he said.
For the Lib Dems, transport spokesman Norman Baker said the government would "betray" its environmental credentials if it proceeded with the project.
"The government has a potentially good story over climate change but it will be shot out of the water if it goes ahead with a third runway," he said.
Earlier, Mr Brown said he expected the number of people who use airports to double in the next 20 to 25 years "because more young people want to travel more, because more elderly people wish to visit relatives and because there is more business travel".
He said less than 10% of Heathrow's traffic was geared towards domestic travel and drew international comparisons with major European airports in Paris, Amsterdam and Frankfurt, which have "either four or five runways there to deal with the traffic".
But Labour MP Martin Salter said the government would have to opt out of its EU obligations on air quality if the runway was built.
"We know that the noise levels are going to be exceeded and quite frankly the environmental case against a third runway is overwhelming," he said.
The Conservatives argue the growth in passenger numbers at Heathrow Airport has been fuelled by international connections, and not visitors to the UK.
A report by the Civil Aviation Authority says 70% of all flight connections in Britain are made at Heathrow, and the majority of these are made by non-UK residents.
But BAA said the benefit of so-called transfer passengers "was clear and simple".
"By filling seats, these travellers underpin international connections, which in turn fuel the UK's global competitiveness by providing fast and reliable routes to market for goods and people," it said.
"The fact that Heathrow's route network is shrinking should concern every politician and only strengthens the need for new runway capacity."