Plans to add a third runway at Heathrow would effectively "create a new airport with the capacity of Gatwick", a court has heard.
Expansion would have "severe" consequences with "increased noise, worsened traffic and harmful air pollution", the High Court was told.
The government's approval for the third runway is being challenged by councils, residents, environmental charities and the mayor of London.
A majority of 296 MPs backed the plan.
'Homes will be demolished'
The case is being brought against Transport Secretary Chris Grayling by five London boroughs, Sadiq Khan, local residents and charities including Greenpeace, Friends Of The Earth and Plan B.
The group argue that the government's national policy statement (NPS) which set out its support for the project had failed to deal with the impact on air quality, climate change, noise and congestion, and has called for it to be quashed.
Outlining the case, Nigel Pleming QC said expansion could see a 60% increase in the number of passengers with an estimated 132 million people using the airport.
He told the court Heathrow was the "busiest two-runway airport in the world" and was situated in a densely populated area meaning expansion would have "widespread consequences".
"Thousands of people's homes will be demolished. Hundreds of thousands will experience increased noise, worsened traffic and harmful air pollution," he said.
Mr Pleming also claimed there were "errors" in the steps leading to the NPS which invalidated it.
Lawyers representing Mr Grayling said the claimants' case was "unarguable" and "premature" as they would be able to make representations during the planning process for the third runway.
When it was being voted on, the transport secretary said the new runway set a "clear path to our future as a global nation in the post-Brexit world".
Construction could begin in 2021, with the third runway being operational by 2026.
The hearing is expected to last for two weeks.