The expansion of Heathrow Airport is expected to be welcomed by Chancellor Rachel Reeves on Wednesday, after years of heated debate.

The move, alongside confirmation of the expansion of other London airports including Gatwick and Luton, is likely to be met by fierce opposition.

Critics say increasing air travel would make it much harder for the UK to meet its climate change commitments.

However, Chancellor Rachel Reeves has told the BBC: “Sustainable aviation and economic growth go hand in hand.”

Heathrow is already the UK’s busiest airport, serving more than 80 million travellers a year with its four passenger terminals and two runways.

A third runway would mean demolishing hundreds of homes, diverting rivers, and rerouting the M25 motorway between junctions 14 and 15 through a tunnel under the new runway.

The number of flights, currently capped at 480,000 a year, could go up to 720,000 – or nearly 2,000 a day on average.

Heathrow told the BBC that it would eventually be able to serve up to 140 million passengers a year once the third runway is in operation.

The airport’s owners, which include Saudi Arabia and Qatar’s sovereign wealth funds and private equity firm Ardian, said the costs of expansion would be met by charging airlines for use of the larger airport.

But after years of wrangling over the original plans, the initial cost estimate of £14bn will need to be revised.

Even with government backing, the formal planning process could take up to two years.

Any planning decision could then be subject to a judicial review, as well as needing to answer questions from opponents over environmental concerns.

Local authorities and nearby residents will also have questions.

After all that, construction is expected to take another six or seven years.

A third runway was first proposed by Gordon Brown’s Labour government in 2009 but was only finally given the go-ahead by the Supreme Court in 2020.

Several members of the current government – including Prime Minister Sir Keir Starmer – voted against a Heathrow expansion when in opposition.

However, recently, there have been hints the Labour government would back a third runway.

Starmer told the BBC that the government has climate commitments, “but growth is really important too.”

Energy Secretary Ed Miliband said: “Any aviation expansion will only be able to go ahead if it is consistent with our carbon budgets”.

However, London Mayor Sadiq Khan’s office said he has “a long-standing opposition to airport expansion around London” because of the impact on air quality and noise.

Heathrow’s boss, Thomas Woldbye, said in December he was waiting for a “clear steer” from the government before he could take plans forward.

Some arguments for investing in the UK’s transport infrastructure include a boost in growth, more jobs, and help for UK exports.

Other European cities boast hub airports with more runways, including Amsterdam which serves fewer passengers than Heathrow but has five runways. Paris Charles de Gaulle and Frankfurt have four.

Currently, over £200bn of British trade passes through Heathrow annually. Heathrow has not devised figures on how much trade is expected to increase after a third runway, which it will do when it arrives at planning stage.

The Airport Commission, an independent panel that examines the need for additional UK airport capacity, looked into the likely impact of Heathrow expansion and reported in 2015 that there would be benefits to the tune of £61bn over 60 years.

It said that included reduced fares, fewer delays, greater resilience, and the creation of up to 77,000 jobs.

Forging ahead with Heathrow expansion would also send a clear message to foreign investors that the government means what it says about making growth its priority.

However, some critics, including think tank the New Economics Foundation say the impact on growth is exaggerated, arguing more capacity in the south of England might simply displace activity from other regions.

Aviation is one source of damaging emissions that can contribute to climate change, and the industry is under pressure to find ways to reduce its impact.

Reeves pointed to progress on creating sustainable fuels, and electric planes. A third runway at Heathrow would also mean “that instead of circling London, flights can land” she added, meaning less fuel is used.

But it is far from clear that those technologies will be enough to make up for the growing demand for air travel. Heathrow will have to prove that its plan meets the governments net-zero by 2050 targets in any planning application.

Last year the government’s independent adviser on cutting emissions, the Climate Change Committee, warned that the UK should not go ahead with airport expansion without a framework in place to manage capacity across the country.

Friends of the Earth has described the plan for a third runway as “hugely irresponsible”.

Alethea Warrington, head of aviation at climate charity Possible, said: “This huge increase in emissions won’t help our economy, and would just encourage the small group of frequent flyers who take most of the flights.”

Plans to expand London’s other airports are already further along than Heathrow’s and have in the past been viewed as an alternative to a third Heathrow runway.

Work is set to start this year to expand capacity at Stansted’s terminal.

The government is due to make a decision on Gatwick airport by 27 February.

Gatwick wants to increase its capacity to 75 million passengers per year, according to previously published plans. Pre-covid, its record was 47 million.

Meanwhile, a decision on Luton airport’s expansion, which includes plans for a new terminal, is due by 3 April.

In total these expansions could create capacity for more than 60 million extra passengers by the 2040s.