London is one of the world’s busiest cities, with around nine million permanent residents. It also has a thriving economy, with a GDP that would put London alone into the top 30 countries worldwide.

A busy city needs strong transport links, and London has a number of airports which carry millions of business passengers and tourists each year. The busiest of these is Heathrow, which handled over 80 million passengers last year. Yet the airport is currently operating at 98% capacity, which means there is little room to process delays or add new routes.

Airlines trade precious take-off and landing spots for millions of pounds, which drives up operating costs and consumer prices. Heathrow is facing the real prospect of overcrowding despite the fact it has two runways, four terminals and 115 gates. Now, though, a £15bn plan has been launched to expand the airport and bring more capacity to London’s busiest transport hub. The new masterplan has a substantial price tag but promises to deliver a new terminal, new roads and taxiways but most importantly, a new runway. At the moment, Heathrow has two and is the busiest dual runway airport in the world, standing in stark comparison to other hubs with greater capacity.

Chicago O’Hare International Airport has the most runways of any civilian airport in the world with eight, while Dallas/Fort Worth International Airport has seven.

Heathrow’s planned third would be 3.5km long and 60m wide, with proposals locating it to the northwest of the current site.

All construction would take place on new land so existing buildings would need to be demolished, electric cables laid underground, and the current land levelled entirely.

There are also plans to divert the busy M25 motorway, which runs around London, into a tunnel underneath the northern runway.

Several rivers and streams would also need to be diverted to deal with the new site.

In place of what is currently on the site, Heathrow has big plans to expand its capacity.

Proposals detail a new terminal, called 5X, which would be connected to the west of the current fifth terminal.

There will be a number of new taxiways built, as well as a direct route from the M25 to Heathrow’s cargo hub.

If all goes to plan, passenger numbers could rocket to 130 million by the end of 2040, but many are concerned about the impact of the development on the environment and the surrounding area.

The plan was ruled illegal by the Court of Appeal in 2020 because judges said it failed to consider the UK’s obligations under the Paris climate agreement, which aimed to limit global temperature increases to 1.5C above pre-industrial levels.

However, this decision was overturned the following year by the nation’s Supreme Court.

There are also major noise pollution concerns surrounding the plans, and critics argue that if they go ahead, some residents will permanently lose current respite time from passing planes.

Heathrow has pledged to ban night flights between 11pm and 5.30am, an upgrade on the current voluntary arrangement not to schedule flights during these times.

Yet London Mayor Sadiq Khan told LBC that the number of people affected by Heathrow’s noise pollution at present is more than that affected by Paris Charles de Gaulle, Amsterdam, Munich, Frankfurt and Madrid airports combined.

At present, Heathrow operates two runaways and alternates them between arrivals and departures to give residents surrounding the site some respite from noise.

A third runway would be operated in “mixed mode”, operating both arrivals and departures.

Under the new plans, the function of each runway would alternate on a 4-day schedule.

After years of protests and delays, Chancellor Rachel Reeves has endorsed the plan, though it remains to be seen whether the project will actually be completed, or even begun.