Prior to the 2010 general election, David Cameron said: “No ifs, no buts, there’ll be no third runway at Heathrow.” Ed Miliband publicly opposed the idea during both his stint as environment secretary and his bid for Labour leadership. And in October last year Liberal Democrat members defied attempts by leaders to amend the party’s policy of “no net increase in runways across the UK” and give it the option of supporting a new runway at Gatwick.

But with the Airports Commission expected to recommend either a new runway at Heathrow, a new runway at Gatwick or an extension of the northern runway at Heathrow when it delivers its final report in June, the clamour of the general election may be swiftly followed by the screech of a political U-turn.

The Department for Transport forecasts [pdf] that passenger numbers will rise from 219 million in 2011 to 445 million by 2050, though market maturity and an expected end to the long-term decline in average fares means growth will slow to between 1% and 3% a year compared to 5% a year over the past 40 years. Much of the increased demand is expected to occur in London and the South-east: airports there will be under substantial pressure by 2030, according to the Airports Commission [pdf]. Heathrow is already operating at 98% capacity, which leads to delays and cancellations during bad weather. However, those who oppose a new runway believe the UK could relieve this pressure by making smarter use of its existing overall airport capacity.

Lobbyists for a third runway at Heathrow, which is expected to cost £18.6bn and will be funded mainly by private investment, claim extra capacity is needed to maintain its hub status in the face of increased competition from the likes of Amsterdam Schiphol, Frankfurt, Paris Charles de Gaulle and Dubai – which recently surpassed Heathrow to become the world’s busiest airport in terms of international travellers. They also say the increase in connections to emerging economies would benefit UK business.

Opponents of the third runway claim demand for point-to-point flights is increasingly making the hub model – airports that airlines use as a transfer point for passengers – irrelevant, and point to the fact that only a third of Heathrow passengers are actually in transit. Analysis carried out by the WWF also found that business flights have been steadily declining for almost a decade. An extra runway at Heathrow would also have a big impact on air quality and noise pollution in West London, and affect roads and rail services in and around the capital.

“Heathrow already subjects more people to unhealthy levels of noise pollution than any other airport, anywhere,” says Zac Goldsmith, Conservative MP for Richmond Park & North Kingston. “Expansion would bring at least 300,000 more people into its noise footprint, and shockingly, none of them has been made aware at any stage of the process.”

A second runway at Gatwick would cost investors an estimated £9.3bn. While it would come with its own set of environmental and infrastructure issues, many believe it would be easier to push through politically. However, the fact that the Airports Commission’s final decision won’t be published until after the election is evidence of a “cosy collusion” between the three main parties to deprive people of any choice in the matter, says Goldsmith. “Sir Howard Davies [chair of the Airports Commission] himself has admitted he could have reported before the election, and it is a democratic scandal that he hasn’t been asked to.”

David Metz, visiting professor in the Centre for Transport Studies at UCL, and former chief scientist at the Department for Transport, says there is a lot of uncertainty about forecasts of future airline passenger numbers. He makes the point that as London’s public transport infrastructure has benefited from constrained road capacity, a constraint on new runway capacity might force the aviation industry to be similarly creative about how it manages demand. Business travellers, he says, might be willing to pay a premium for access to a centrally located airport like Heathrow, shifting leisure travellers – who are more likely to put up with extra travel time to save money – to other airports like Stansted and Gatwick.

Crucial environmental questions also remain unanswered, according to the Aviation Environment Federation (AEF). The environmental analysis the Airports Commission committed to undertaking has not been finished in time, it says, which means there are holes in its analysis of airport expansion – especially in terms of local air quality modelling and meeting the UK’s commitment to reduce greenhouse gas emissions by 80% by 2050.

It claims the Airports Commission’s own work has shown that building a new runway would be inconsistent with UK climate change commitments unless new, unspecified action was taken by government to cap aviation emissions. Cait Hewitt, deputy director of the AEF, says analysis of how any carbon cap on airline emissions would be delivered has been a policy gap for some time. “It’s just not anything that anyone wants to talk about, because the only things are measures like big increases in air passenger duty – which is obviously going to be politically unpopular. And constraints on growth at regional airports, which would also be extremely unpalatable.”

The Civil Aviation Authority (CAA) says that the airline industry will need to do more in future to address the concerns of those opposed to new runway capacity, including using the most quiet aircraft possible, operating them in a more noise efficient way, such as using steeper landing approaches, and mitigating noise through compensatory schemes. Gatwick recently announced that a £1,000-a-year payment would be available for those most affected by noise from a second runway, while Heathrow has proposed a £700m expansion of its noise insulation scheme should a third runway be developed.

Nic Stevenson, senior policy analyst for the CAA, says these approaches are best debated through a community engagement forum that brings together local communities, the aviation industry and policymakers, and that the Airports Commission should recommend that one is established when they deliver their final report. “We’ve not been very successful when it comes to developing new runway capacities, despite having a thriving – world-beating if you like – aviation sector,” he says, which is in part due to the industry’s “failure to effectively engage and compensate communities, and develop community trust.”

More in the series on transport policy post May 2015:
• The road ahead: how will the election result affect UK drivers?
• Full steam ahead: how will the election result affect UK rail passengers?

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