More doubt has been cast on the prospect of the so-called "Boris Island" Thames Estuary airport plan going ahead.
Favoured by London Mayor Boris Johnson, the estuary scheme, involving a new four-runway airport, is being studied by the Whitehall-appointed Airports Commission.
One of three reports prepared for the commission published today has said of the estuary plan: "Overall, the challenges to transition are considerable and amount to a significant cost and risk to the taxpayer in terms of commercial negotiations, infrastructure development and potential failure."
More top news
Pictures of today's events unfolding following a major security incident at the Houses of Parliament.
At least two people have died after a knifeman brought terror to the heart of Westminster.
The Cobra committee brings together government ministers with senior officials of the emergency services and security agencies.