Plans to build new runways at Heathrow and Gatwick will cost substantially more than the bidders have estimated, a report says.
The Airports Commission says a second runway at Gatwick would cost £2bn more than the bid suggests.
Two separate plans to expand Heathrow are predicted to cost £3-4bn more.
The commission has been tasked with weighing up where a new runway should be built to meet rising capacity demand in the South East of England.
