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.

The third runway is part of its drive to lift the U.K. economy out of a long period of stagnation, Treasury chief Rachel Reeves said in a speech on Wednesday.