The government has approved the construction of a third runway at Heathrow, following more than a decade of discussion, in order to increase the number of flights able to use the airport.
The decision was made by a cabinet committee on Tuesday morning, although it will still be years before construction actually begins and there are many legal and political hurdles before the decision is formalised.
Environmentalists oppose the expansion on the grounds that it will increase air and noise pollution in west London.
The decision saw the government reject rival proposals to expand an existing runway at Heathrow or to build a second runway at London's Gatwick airport.
"We think this is the right decision for Britain," said transport secretary Chris Grayling, who said the expansion would boost the UK economy by £61 billion. "I am proud that after years of discussion and delay this government is taking decisive action to secure the UK's place in the global aviation market."
Tory MP Zac Goldsmith promised to resign and trigger a parliamentary by-election if the government allowed the Heathrow expansion to go ahead. Shortly after the announcement, his office confirmed he would honour the pledge.
Cabinet ministers Boris Johnson and Justine Greening, longterm opponents of the plan, are expected to criticise the decision after being given permission to publicly break with government policy by the prime minister.
Theresa May, whose Maidenhead constituency sits under the Heathrow flight path, has herself previously criticised expansion. In 2009 she told the House of Commons that "we need a better Heathrow, not a bigger Heathrow".
Hasn't this been announced before?
Yes, governments absolutely love to announce the expansion of Heathrow. It's one of their favourite things. Alistair Darling announced outline plans for a third runway at the airport in 2003. The Labour government reaffirmed this commitment in 2007 and the House of Commons approved expansion in 2009.
The Conservative-Lib Dem government scrapped the project when it came to power in 2010, only for the coalition to change its mind in 2012 and launch an independent Airports Commission to re-examine the issue.
In 2015 it recommended building a third runway at Heathrow, which Theresa May's Conservative government has now decided to back. This essentially leaves us back where we were a decade ago.
Will the runway actually get built this time?
The government insists that it will, although even the most optimistic predictions suggest it will take until the late 2020s before planes are using the new runway at Heathrow.
There will now be a public consultation on those affected in the local area, then the government will produce a national policy document. This will then be approved by a vote in parliament – most likely in late 2017.
There are also set to be endless environmental audits, plus substantial potential for legal challenges from opponents of the plan.
Only once all those issues are covered can the airport apply for formal planning permission to build the runway. This would then need to be approved by the government once again. That's why Heathrow will be lucky to start work on building the runway by 2020.
What are the political consequences of expansion?
Many prominent Conservative MPs have constituencies under Heathrow's flight path and have campaigned strongly against expansion. Foreign secretary Boris Johnson spent years as mayor of London pushing an alternative vision of an airport on an island in the Thames Estuary, while education secretary Justine Greening has repeatedly campaigned against the proposal. Labour leader Jeremy Corbyn opposes Heathrow expansion.
Jim Waterson is a politics editor for BuzzFeed News and is based in London.
Contact Jim Waterson at firstname.lastname@example.org.
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