Expansion plans at Heathrow and Gatwick will cost considerably more than previously forecast, according to the Airports Commission's latest analysis of the airports' bids.
Plans for a second runway at Gatwick airport could cost £2 billion more than its bid suggests and the two separate schemes at Heathrow are predicted to cost £3-£4 billion more.
The suggestions come as a 12-week consultation is launched which is inviting public comment on the Commission's analysis of each of the three proposals.
The head of the commission, Howard Davies, said no decision had yet been made.
"We have not yet taken a view on which proposal strikes the most effective balance between the assessment criteria," he said.
"It is important first that we provide an opportunity for this evidence to be examined, challenged and improved. This consultation gives everyone with an interest in the issue of airport expansion that opportunity."
According to the commission's analysis, Gatwick's plan to add a second runway is the quietest and easiest to deliver.
But, expanding Heathrow is seen as more likely to deliver a bigger boost to the economy, and create more jobs.
The consultation will run from November 11 until February 3 2015.
Read an analysis of the Davies commission’s interim report for UK airports.