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.

British finance minister Rachel Reeves is expected to back the expansion of Heathrow Airport on Wednesday, turning to the country's most controversial infrastructure project in her hunt for economic growth.