Heathrow Airport has launched a consultation into expansion including suggestions to move the M25 into a tunnel to go under a third runway.
The owners have put various proposals to the public ahead of a vote in Parliament later this year.
The Department for Transport has previously warned London's five airports would reach full capacity by 2034 without expansion.
Airlines said moving the motorway could cause "years of disruption".
Heathrow Airport said one plan could move the M25 to the west by 150m.
The motorway could be lowered by 7m to create a tunnel between junction 14 and junction 15, with the runway on top.
But British Airways' parent company, International Airlines Group (IAG), said the airport must provide a detailed risk and cost analysis of its proposed M25 changes "which could derail expansion plans and bring years of disruption on a motorway plagued by traffic jams".
And Rob Barnstone, from the No 3rd Runway Coalition representing MPs, local authorities and campaign groups opposed to expansion, said the proposals "lack" explanation and "robust plans to address issues arising from expansion including air quality and roads and rail around the airport".
The consultation also suggests:
- Possible locations for new or extended terminals
- Three options for a new runway's length
- Principles for designing new airspace
- Cutting expansion costs by £2.5bn
Future flight paths are not yet being consulted on, however.
Cutting costs will contribute to ensuring that landing charges stay "close to today's levels" of about £22 per passenger, Heathrow said.
An IAG spokesman said Heathrow needs to prove "passengers are not ripped off".
Virgin Atlantic said it would scrutinise the proposals as "cost and efficiency remain huge concerns".
Anti-expansion campaigners said there is still a chance to call off the development.
Cait Hewitt, from the Aviation Environment Federation, said: "No one should assume that a decision to expand Heathrow is done and dusted."
There will be a second, more detailed, consultation next year before the airport submits its application for planning consent.
Emma Gilthorpe, Heathrow's executive director of expansion, said: "We need feedback to help deliver this opportunity responsibly and to create a long-term legacy both at a local and national level."