The third runway at Heathrow Airport could involve planes taking off from a "ramp" over the M25 motorway, the transport secretary says.
Chris Grayling said this would be "cheaper and quicker" than building a tunnel for the M25 under the new runway and would cause less disruption for drivers during construction.
Theresa May defended the government's plans to expand Heathrow during PMQs.
The prime minister told MPs the scheme could meet air pollution standards.
She was responding to a question from Tania Mathias, one of the Conservative MPs fighting the third runway.
The government's long-awaited backing for a third runway at London Heathrow has also been attacked by cabinet ministers Boris Johnson and Justine Greening and sparked the resignation of Conservative MP Zac Goldsmith.
The decision is also set to face a challenge in the courts, with the Richmond council leader Lord True telling BBC Radio 4's Today programme he was "taking legal advice".
The Conservative peer said Heathrow was "busting air quality legal limits" and was responsible for "40% of all noise pollution in Europe associated with airports".
He added: "The fact that the government has already delayed action for a year results from our reminding them that they hadn't yet fulfilled things which they're required to do under the existing law."
Lord True said he would be campaigning for Mr Goldsmith, who will contest the Richmond Park by-election as an independent with the Conservatives not putting forward a candidate against him.
The Department for Transport says the new runway will bring economic benefits to passengers and the wider economy worth up to £61bn and create as many as 77,000 additional local jobs over the next 14 years.
But Ms Greening said she was "extremely disappointed", while Mr Johnson said there were "colossal" problems with the proposal and anticipated it would be "snarled up" in legal cases.
Interactive See how proposed flightpaths with the third runway differ from current flightpaths
Proposed Flight Paths
Current Flight Paths
A public consultation will now be held on the effects of airport expansion before the government puts the decision to the House of Commons as part of a National Policy Statement on aviation.
MPs will then vote on that decision in the winter of 2017-18. It is unlikely that any new runway capacity would be operational before 2025.
Mr Grayling added that the construction of the third runway would be "difficult for people who live close by", and said he was "sorry" Mr Goldsmith had decided to resign his seat.
But he added: "Ultimately in politics, you have to do what's best for the whole of the United Kingdom."
Responding to a question about how the runway would bypass the M25, he said many other airports around the world had built runways over motorways.
"It is a cheaper and quicker way of doing it. I am, of course, very concerned to make sure that, as this runway is built, it doesn't cause massive disruption on the M25, so I think this is a sensible way.
"It is a very gentle hill up which the planes would take off rather than a flat surface. It's what happens at very many airports around the world."