Building new runways at Heathrow and Gatwick airports will cost substantially more than the bidders have estimated, according to a new report.

The Airports Commission has revealed a second runway at Gatwick would cost around £2 billion more than the bid stated, while building a third runway at Heathrow would cost almost £4 billion more than originally estimated.

A consultation has been published by the commission, providing assessments of the business cases for expansion at the airports. It has already ruled out the construction of a new hub airport in the Thames Estuary.

The cost of construction at Gatwick could lead to a doubling of the current airport charges to an average of £15-18 per passenger and a peak of £23. Heathrow's charges would rise from £20 to £28-29 and could peak at £32, the report states.

Airports Commission chairman Howard Davies said: "Responses to this consultation will be a valuable addition to our evidence base and will directly inform our recommendation to the government when we publish our final report in the summer of 2015."