Runway expansion plans could be far more expensive than those bidding to operate them have estimated, the Whitehall-appointed Airports Commission said yesterday.

A new north-western runway atHeathrow would cost around £18.6 billion as opposed to Heathrow Airport Ltd’s estimate of £14.8bn, the commission said.

Another plan for Gatwick, estimated by Gatwick Airport Ltd to cost £7.4bn, would cost an estimated £9.3bn, the commission added.

And the third shortlisted option — an extension of the existing northern runway at Heathrow — would cost an estimated £13.5bn, the commission said, higher than the £10.1bn 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.