Heathrow has kicked off a 10-week public consultation on its expansion plans, which include proposals to build a tunnel for the M25 beneath the new third runway.
The public’s views are also being sought on the length of the controversial third runway at the UK’s biggest airport.
The government, which first approved Heathrow expansion in 2016, is aiming to give the go-ahead to the new runway in the first half of this year.
Heathrow’s consultation includes plans to move the M25 150 m to the west and lower it by seven metres into a tunnel, so the new runway could pass over it.
It had previously been suggested that Heathrow could consider scrapping the M25 tunnel plan to reduce costs.
Three options are also being tabled for the length of the runway, between 3,200 m and 3,500 m.
Critics of a new runway have raised concerns over the potential impact on air quality and noise levels on local communities.
The cost of the proposed expansion has also sparked fears, but last month airport chiefs said they will look to cut the price of the new runway by £2.5bn to £14bn.
“Heathrow is consulting to ensure that we deliver benefits for our passengers, businesses across the country but also, importantly, for those neighbours closest to us,” said Heathrow executive director expansion Emma Gilthorpe.
Other proposals include moves to expand terminal capacity, either east of Terminal 2, west of Terminal 5 or a new satellite terminal by the new runway.
The consultation, which closes on 28 March, also asks for views on the potential principles, or ‘rules’, that could apply when designing the new airspace required for an expanded airport.