The Court of Appeal is due to rule on the plan to build a third runway at Heathrow to increase the number of flights to and from the London airport.
The idea to expand Heathrow has been talked about for many years and while many businesses support it, the plan has also attracted a lot of opposition from environmentalists and local residents.
So what exactly is the third runway proposal and what are the arguments for and against it?
What is the Heathrow expansion plan?
Heathrow is already the UK’s busiest airport, serving about 80 million passengers per year. The airport currently has four terminals and two runways.
If it gets the green light, construction of a new runway is expected to be completed between 2028 and 2029.
Last summer, Heathrow Airport had set out an earlier completion date of 2026 under its “master plan”. However, the airport was forced to revise its timetable after the Civil Aviation Authority – the aviation regulator – warned that the “aggressive schedule” could leave passengers shouldering the cost.
Building the new runway would involve diverting rivers, moving roads and rerouting the M25 through a tunnel under the new runway.
But that would not be the end of the project. Work is expected to continue until 2050, with extensive upgrades to the existing terminals two and five as well as plans for new car parks.
Heathrow Airport says the project will be funded privately.
What are the arguments for a third runway?
More people are travelling by plane than ever and that’s left the government looking for ways to expand airport capacity.
The CBI – which represents UK businesses – says a third runway will “set the UK on course for a bright trading future”.
The expansion will also benefit passengers, boost the wider economy by up to £61bn and create up to 77,000 local jobs by 2030, according to the Department for Transport.
More than 40% of the UK’s exports to non-EU countries now go through Heathrow, according to its chief executive John Holland-Kaye. He says a third runway is vital in order to strengthen international trade links.
“If we don’t expand our only hub airport, then we’re going to be flying through Paris to get to global markets.”
Heathrow Airport also says it will introduce legally binding environmental targets – including on noise, air quality and carbon emissions.
What are the arguments against?
Local and environmental groups have dismissed Heathrow Airport’s assurances and say a new runway would mean unacceptable levels of noise and pollution, as well as adding to the UK’s carbon emissions from the increased number of flights.
The proposal also “makes a mockery” of the government’s 2050 carbon neutral strategy, according to Green MP Caroline Lucas.
Rupa Huq – the Labour MP for neighbouring Ealing Central and Acton – has labelled the plan “completely nuts” and says “Heathrow is the biggest emitter of carbon dioxide in Europe”.
Campaign groups have also voiced opposition.
“What hits you is the scale of these proposals,” said John Stewart, who chairs the Hacan group.
“The impact on local people could be severe for many years to come. Disruption from construction, the demolition of homes, the reality of more than 700 extra planes a day.”
In all, 761 homes are expected to go, including the entire village of Longford.
Heathrow says it will pay the full market value plus 25% for properties in its compulsory purchase zone, as well as for some houses in the surrounding areas.
What does the government think?
The third runway issue puts Prime Minister Boris Johnson in an awkward position – especially as his Uxbridge and South Ruislip seat is next to Heathrow.
In 2015, he said: “I will lie down with you in front of those bulldozers and stop the construction of that third runway.”
However, when MPs voted in favour of the third runway in 2018, Mr Johnson – who was foreign secretary at the time – missed the vote as he was travelling to Afghanistan.
Mr Johnson is not the first PM to grapple with the Heathrow expansion issue.
The Labour government first gave the third runway the green light in 2009, with former Prime Minister Gordon Brown saying it was needed for economic reasons.
But the plan was later scrapped by the Conservative-Lib-Dem coalition government in 2010. David Cameron, who became prime minister after Mr Brown, had previously ruled out a Heathrow expansion “no ifs, no buts“.
A few years later an Airports Commission was set up to look at how to deal with London’s airport capacity problems. In 2015, it recommended Heathrow as the preferred site for a new runway.
A year after the commission’s report was published, the government approved the plan under Theresa May’s leadership.
However, the decision caused some disquiet within the Conservative party. Zac Goldsmith – the MP for Richmond Park – resigned his seat in protest.
The Conservatives’ 2019 election manifesto says that while the third runway is a private project, Heathrow must demonstrate it can meet its air quality and noise obligations.
What happens next?
Heathrow Airport is currently reviewing responses to a recent public consultation.
A finalised plan will then need to be submitted to the planning inspectorate – a public body that makes decisions on big infrastructure projects. A final decision to approve it will then be made by the transport secretary.
The government is also facing a legal challenge. Campaigners argue the proposal is unlawful because it does not take into account the Paris climate change agreement to limit global temperature rise to under 2C.
So, depending on the outcome – there’s still no guarantee the third runway will go ahead.