ON JANUARY 10th 1946 Britain’s cabinet approved outline proposals for a third runway at Heathrow, London’s main international airport. When Heathrow’s first two runways opened at the start of that month, the airport was little more than a muddy airfield located 15 miles (24km) to the west of the city centre. Passengers had to make do with a row of tents to check-in. It has since become one of the world’s busiest airports. This year 75m people will pass through its humongous new terminals, which handle nearly a third of all passengers using Britain’s airports. But although Heathrow’s buildings have been transformed and modern jetliners have replaced little propeller planes on its taxiways, the airport still has only its original two runways. On October 25th Britain’s new government announced its support for a revised proposal for a third runway at Heathrow. But why has it taken more than 70 years to start building the first full-scale runway in south-east England since the second world war?

Britain’s government knows its capital city needs more airport capacity. Since 1946 more than a dozen commissions, policy documents and white papers have investigated where to put a new runway near London. But all the proposals they came up with have been defeated by NIMBYs, tight budgets and legal challenges: at Heathrow in the 1940s; Cublington in Buckinghamshire in the 1960s; Foulness in the Thames estuary; Gatwick, south of London, in the 1970s; and at Heathrow again in the 1990s and 2000s. Yet intense local opposition to airport expansion almost anywhere in the south-east, on the grounds of aeroplane noise, air pollution and increased traffic on roads nearby, is not new. The original runways that became Heathrow were built using emergency wartime powers because local opposition would have meant they would not have received planning permission otherwise, says Philip Sherwood, a historian of the airport.

As a result of seven decades of political dithering about where to put a new runway, the need for more capacity is now urgent. Heathrow’s two runways have been operating at 99% of capacity for the past decade, while Gatwick’s single landing strip is now full for most of the day too. In 2012 David Cameron, then the prime minister, handed the problem to a commission of experts led by Sir Howard Davies in an effort to take the decision out of the hands of politicians. Sir Howard shortlisted three options: a new airstrip to the north-west of Heathrow, a runway extension there, or a new runway at Gatwick. Swayed by the wider choice of destinations offered by a bigger Heathrow, last year he recommended it should get a third runway (see map). But Mr Cameron delayed making the actual decision three times, due in part to environmental problems with Heathrow’s proposals and also to avoid annoying voters before London’s mayoral election last May and the EU referendum this June.

And so the choice has fallen to Britain’s new prime minister, Theresa May. Once an opponent of expanding Heathrow, she is now pushing through plans for a third runway. By pressing on with Heathrow, which most businesses favour over Gatwick, she will boost her pro-enterprise credentials, which have been damaged by her hardline approach to Brexit. And with no general election due until 2020, and Labour hobbled by an unpopular leader, she can afford to displease voters in the marginal constituencies under Heathrow’s flight path (as it happens, planes using the new runway will fly directly over her own constituency). And Brexit may have strengthened the case for Heathrow. A third runway would help the airport offer more routes to cities in fast-growing economies such as China and India, with which Britain will need more trade after leaving the EU, argues Heathrow’s boss, John Holland-Kaye. But although Heathrow’s plans were given the green light today, work on the new runway is unlikely to start before 2020, which means it could not open until 2025. And a legal challenge from Gatwick or Stansted, an airport north-east of London, angry that they have been passed over for a new runway, may delay that timetable further. London’s 70-year wait for a new runway may not be over yet.

Update: This blog post was amended on October 25th to reflect the British government's decision to approve a third runway at Heathrow airport.