New runways at Gatwick and Heathrow could cost up to £3.4 billion more than the airports claim, according to a report from the airport commission.

In an assessment of the options to increase airport capacity in the south east of England, three options have been considered – an extension to an existing Heathrow runway, a second runway at Gatwick or a third runway at Heathrow. Other options including a new airport near Oxford have been ruled out.

The commission said that a third runway at Heathrow could cost up to £3 billion more than anticipated, whilst the option to extend its current northern runway could cost up to £3.4 million more. Meanwhile, a second runway at Gatwick would cost £9.3 billion rather than the £7.4 billion originally estimated.

The head of the commission, Sir Howard Davies, said that the increase in costs was likely to escalate the current airport charges at both airports, which would hit Gatwick particularly hard thanks to its low-cost airline base.

Consultation on the options for airport expansion is due to end in February, with the commission’s recommendation due after May’s general election.