The cost of Boris Johnson's proposed floating airport in the Thames Estuary could be £70 billion - more than double previous estimates.

The admission has come from Daniel Moylan, Transport for London's deputy chairman and the man proposing the controversial idea.

He said: "We haven't yet worked out a recommendation for a particular location and one of the biggest costs when building an airport is the access to it, the cost of which will depend on where it is built.

"A very rough estimate might be between £40 to £60 or £70 billion."

He said it was a 20-year project, which made spreading the cost "a little bit less frightening".

Sheppey MP Gordon Henderson is dismissive of the idea and said Prime Minister David Cameron would not support it.

London Mayor Mr Johnson has toned down his original scheme - dubbed Boris Island - for two islands and six runways to one with four runways.

But opponents say it would still cause massive pollution and destroy areas of world ecological importance.

Medway Council, backed by Kent County Council, says the estuary airport is undeliverable, unaffordable and unnecessary.

The four-runway island airport could be a replacement for Heathrow, but many airlines are opposed to it - with Virgin last week saying the scheme was the wrong side of London.