Heathrow Airport and rival Gatwick have published revised plans for new runways to increase London's airport capacity, marking the latest stage in a long-running debate over how and where the expansion should take place.
Heathrow, Britain's busiest airport, is competing against Gatwick, the second busiest, plus a second expansion option at Heathrow proposed by Heathrow Hub.
These three shortlisted groups revealed their latest proposals on Tuesday, with Heathrow Airport and Gatwick broadly sticking to earlier cost estimates.
Heathrow Airport said a third runway could now be delivered by 2025, a year earlier than the group estimated last July and bringing it into line with Gatwick's time frame.
Britain's Airport Commission is due to make a final recommendation on where and how to expand London's airport capacity by next summer after a general election.
Politicians and business leaders have said south-east England needs new runway capacity to help the country remain economically competitive. But runway expansion is unpopular and a decision has been repeatedly pushed back.
Passions ran high during Britain's last election in 2010 over possible expansion at Heathrow due to its proximity to London and the safety and pollution issues this would create.
Plans to expand Heathrow were scrapped in 2010, but in December two expansion options for Heathrow were shortlisted, marking a significant u-turn.
The Commission has also said a plan to build a new airport on the Isle of Grain to the east of London in the inner Thames estuary remains an option, but this is not formally on the shortlist. The Commission will make a decision about whether to include it later this summer.
The cost of Heathrow's plan would be 16.8 billion pounds ($28.34 billion), inclusive of a 1.2 billion pound bill for the government for new road and rail access costs. The balance would be met by debt and equity investment from its investors.
Gatwick Airport, which said in March that it could deliver a new runway by 2025, estimated the cost of its proposal at 7.8 billion pounds. It said there would be no additional cost to government.
Heathrow Hub, led by consultant Mark Bostock and former Concorde pilot William "Jock" Lowe, estimated the cost of its plan, which involves extending one of Heathrow's existing two runways, at about 12 billion pounds, financed by private investment. Heathrow Hub said that the runway extension could be completed by 2023.
Gatwick said its proposal would bring Britain economic benefits of about 80 billion pounds as a new runway at Gatwick would increase competition and bring lower fares. Heathrow estimated it would boost Britain's economy by 100 billion pounds. Heathrow Hub said it would deliver benefits of 45 billion pounds.
Heathrow is owned by unlisted Heathrow Airport Holdings which is co-owned by Spanish infrastructure firm Ferrovial and other partners.
Gatwick Airport is owned by a group of international investment funds ultimately controlled by funds managed by a unit of Global Infrastructure Partners (GIP).