The Mayor of London Boris Johnson says his plan for an off-shore airport off the Kent coast must not be discounted and would pose no more risk to birds than Heathrow airport.

In fresh comments likely to risk further anger among opponents of his idea, Mr Johnson said it was "time for vision" and an estuary airport must be considered if the region was to compete with its European rivals.

Defending the estimated £40bn project, he said it was "surely right" to consider it seriously.

"We cannot, to coin a phrase, go on like this. It is time for vision."

His latest remarks were quickly denounced by Sittingbourne and Sheppey MP Gordon Henderson.

He said: "To continue to argue that it is a viable proposition is totally lunatic.

"The government does not support it; local councils don't; airlines don't want it and neither do environmentalists."

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In an article for the Daily Telegraph, Mr Johnson also took a side-swipe at David Cameron's policy for more high-speed rail links, saying trains will end up stopping so often that "you will end up with low speed trains on high speed tracks".

Mr Johnson insisted that the current policy of building no more runways at airports in the south east was "utterly ridiculous".

He said: "That is why some people are arguing for a clean, green 24-hour hub airport that could be built in the Thames estuary, far from human habitation, with no more threat to bird life than there is at Heathrow.

"I don't know if they are correct. But we are surely right to look at it seriously."

He went on: "Planes are becoming ever cleaner and greener.

"In 1985 the average passenger aircraft used eight litres of fuel per passenger per 100km.

"That is now down to three litres, and falling, and in the next 20 years we will have vast flying wings, capable of carrying 1,000 passengers and saving 40 per cent on fuel.

"Wouldn't it be utterly insane if they can land everywhere else in the world except Britain?"

Even if the Government was so "mad and bad as to break its word and build a third runway at Heathrow, that would still not be enough".

Transport minister Philip Hammond recently told MPs in Kent that the airport plan formed no part of the coalition's aviation policy.