London Gatwick Airport is London's second largest airport and the second busiest airport in the United Kingdom after Heathrow. It is the world's 22nd busiest airport in terms of passengers per year (7th in terms of international passengers). It is also often quoted as the world's busiest single runway airport, although strictly speaking it now has a second 'stand-by' runway, which can only be used when the main runway is out of use. 