By political editor Paul Francis

Lord Adonis has sunk plans for a huge floating airport in the Thames estuary off the north Kent coast.

The transport minister dubbed London Mayor Boris Johnson's ambitious scheme as "not in any way credible".

He said the plans were flawed and the prospects of it ever getting off the ground remote.

"The idea that you could have permission to start advancing this as a serious proposal next year is clearly for the birds. We don't regard it as in any way credible".

He cast serious doubt on the funding, saying: "Where is the £40bn going to come from? Has Boris got any idea where he's going to magic up £40bn?"

He also questioned whether the proposals were consistent with current aviation policy.

"Where is the consensus that in fact this is the way we should move in terms of a big introduction of additional airport capacity ... even incremental proposals for increased capacity at the moment are very controversial".

"The problem there is we're starting from where we are. Immediately post-war it would have been very sensible to build an airport in the Thames Estuary. Indeed all the way through until the 'seventies this was regarded as a credible option."

Deputy mayor of London Kit Malthouse recently claimed funding for the scheme could come from international investors.

He said the Boris Island project had drawn the interest of middle Eastern countries as well as China.

Mr Malthouse said: 'We have had an incredible amount of interest from countries such as Kuwait, Qatar and the UAE, which have signalled they would like to fund the airport. It is possible we could build it without taxpayers' money."

A review has been completed and an engineering company has been carrying out a feasibility study commissioned by Mayor Johnson.

Council chiefs in Kent and Medway have re-iterated their opposition to the scheme. KCC leader Cllr Paul Carter said: "There is a growing consensus that the Estuary airport is undeliverable, unaffordable and unnecessary."