Runway expansion plans could be far more expensive than those bidding to operate them have estimated, the Whitehall-appointed Airports Commission said today.
A new runway at Gatwick, estimated by Gatwick Airport Ltd to cost £7.4 billion, would cost an estimated £9.3 billion, the commission said.
A new runway to the north-west of Heathrow would cost around £18.6 billion as opposed to Heathrow Airport Ltd's estimate of £14.8 billion, the commission added.
And the third option shortlisted by the commission - an extension of the existing northern runway at Heathrow - would cost an estimated £13.5 billion, the commission said.
This is higher than the £10.1 billion estimated by the scheme's promoters, Heathrow Hub, a consortium which includes former Concorde pilot Jock Lowe.
The commission also estimated that the increases to airport costs, based on the cost per passenger, would be higher as the result of each scheme than those forecast by the bidders.
The cost estimates from the commission came as it published, for public consultation, its latest assessments of the three schemes.
The commission, chaired by Sir Howard Davies, is due to make its recommendation on which scheme should go ahead when it delivers its final report in summer 2015.
Launching the consultation today Sir Howard said: "Since our interim report last year we have undertaken a huge amount of work. We have carried out a thorough assessment, across a comprehensive range of subjects, looking at the benefits and impacts of each proposal.
"We have not yet taken a view on which proposal strikes the most effective balance between the assessment criteria. 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.''
the commission said a second runway at the West Sussex airport would cost an estimated £9.3 billion which, it added, was higher that Gatwick Airport Ltd's estimate of £7.4 billion.
The new runway would be built south of the present runway, and there would be a new terminal building between the two runways with a capacity of about 50 million passengers a year, slightly higher than the combined capacity of 45 million of the current North and South terminals.
A total of 168 homes would probably have to be demolished and further residential properties could also be lost, the commission said.
Investment in the new runway would, in the commission's view, lead to increased airport charges of between £15 and £18 per passenger with peak charges of up to #23.
The commission said these figures were higher than Gatwick's own estimates of charges of £12 to £14 per passenger compared with the current £9.
The commission said: ``The delivery risks associated with the Gatwick scheme are assessed as relatively low and the commission considers an opening date of 2025 is achievable.''
Expansion would mean an increase in annual flights from 280,000 at present to 560,000.
On the Heathrow Hub plan, the commission said it assumed that the hub scheme would be purchased, delivered and financed by Heathrow Airport Ltd, with the commission listing the same financial concerns as it in its assessment today of Heathrow's own north-western runway scheme.
The commission said delivery risks associated with an extended runway at Heathrow were substantial'' but "could be managed''.
The Hub plan would see the number of annual flights going up from 480,000 to 700,000.