[5] The proposal met with strong opposition from local people and more broadly from politicians and middle-class voters which made it politically untenable.

[6] One influential member of the Roskill Commission, Colin Buchanan, dissented on environmental and planning grounds and proposed an alternative site at Maplin Sands, Foulness, in the Thames Estuary.

[7] This opened the door to strong political opposition against Cublington and in April 1971 the government announced that the site at Maplin Sands had been selected for the third London airport, even though it was the most remote and overall the most expensive of the options considered, and that planning would begin immediately.

The project would have included not just a major airport, but a deep-water harbour suitable for the container ships then coming into use, a high-speed rail link together with the M12 and M13 motorways to London, and a new town for the accommodation of the thousands of workers who would be required.

[12] The scheme was abandoned in favour of a cheaper plan to enlarge Stansted rather than building an entirely new airport; the requirement for a container ship harbour was to be discharged by the development of Felixstowe.

[15] In December 2003 the government decided against the Cliffe proposal on the grounds that the costs of a coastal site were too high, and there was a significant risk that the airport would not be well used.

The "Marinair" proposal was put forward in the 1990s,[17] in which an airport would be built on an offshore artificial island in the Thames estuary, north east of the Isle of Sheppey.

The Isle of Sheppey proposal was revived in 2008 by Mayor of London Boris Johnson, located a little further to the east towards the Shivering Sands area, north-east of Whitstable.

[18] In November 2008 the mayor appointed Doug Oakervee (executive chair of Crossrail) to lead the Greater London Authority's preliminary feasibility study[19] which determined in October 2009 that there is "no logical constraint" to the plan.

[26] The scheme was proposed by Testrad (Thames Estuary Research and Development), initially an agency formed by Johnson but now also involving other partners, and was rejected by the airport commission in January 2014 in favour of continued upgrades to Heathrow.

[27] Proponents argue the scheme's big advantage is that it would avoid flying over densely populated areas and the noise pollution and other problems that causes.

[29] Critics suggest the scheme is impractical and too expensive; Terry Farrell compared it to grandiose and unrealistic projects devised by Nazi leader Adolf Hitler.

Thus it would reassert London's geographical advantage as the stop-off point between North America and Eurasia, which is being eroded by a combination of new long-range aircraft and the emergence of networks centred on a global hub, such as Dubai.

It was finally rejected on grounds of cost (possibly as high as £100 billion) and environmental damage by the Airports Commission in an announcement made on 2 September 2014, leaving Gatwick and Heathrow as the remaining options.