BORIS JOHNSON, the London mayor, has unveiled detailed proposals for a £40 billion airport spanning the Thames estuary in a move aimed at presenting a credible alternative to the government’s plans for expanding Heathrow.

Early findings from a study by the engineer behind Hong Kong’s island airport suggest that a four-runway airport is both technically feasible and would serve Britain better. It could be built in eight years, he said.

The bold scheme entails splitting the airport in two, with runways placed on two separate islands in the mouth of the Thames.

Passengers would shuttle between the islands in a tunnel below the river bed, running from Essex on the north bank, to Kent on the south. Douglas Oakervee, who masterminded the engineering of Hong Kong’s international island airport in the 1990s, said that splitting the airport in two would reduce disruption to local wildlife. It would also enable the airport to connect to high-speed rail routes to the Continent.

Underwater turbines, built into ducts running through the body of the islands, would generate nearly all the airport’s electricity needs by harnessing the tide .

The scheme would be “simpler to build than Hong Kong”, Oakervee, the study’s lead engineer and chairman of Crossrail, said on a boat trip to inspect the site. “The engineering aspect of it would be relatively simple. In Hong Kong we had to flatten two islands and the sea was very deep. Here it’s just 15 metres or so.”

Johnson has chosen to make public his vision for an alternative “hub” airport for the capital as MPs prepare to debate the future of Heathrow in the Commons this week – two weeks after the government approved a third runway. Speaking to The Sunday Times aboard a dredger, Johnson vowed to continue to oppose the expansion of Heathrow. He also confirmed that he aims to mount a legal challenge against the government’s decision within weeks.

Lawyers representing the 2M Group of residents in west London, whose legal costs are being part-funded by city hall, are now studying the decision to see if there is a case for a judicial review.

Although Johnson has described Heathrow as “a planning error of the 1960s”, his advisers believe it could continue to work with two runways even if the new hub is built.

The two estuary terminals would be served by road and rail links. The larger terminal, in Kent, would be connected to Crossrail and the high-speed Channel tunnel rail link, whisking passengers to central London in about 35 minutes. The rail connections to Europe would cut out the need for many short-haul flights.

Because flights would take off and land over water, they would cause relatively little disturbance to the nearby towns of Sheerness and Southend-on-Sea. According to Oakervee, the location in the estuary, rather than on the mud flats, means the risk of bird strikes would be low.

Johnson said he felt “reassured” that the scheme was practical. “Coming here has put paid to talk of a fantasy island. You get a sense of just how far the airport would be from the shore.

“I’m convinced that this is an option we should look at seriously and the government’s decision on Heathrow makes it all the more urgent that we came up with alternatives.”

The £40 billion price tag would include the cost of extending the high-speed rail network, widening and extending the nearby M2 and extending Crossrail to the Kent terminal from southeast London. It compares with a £13 billion estimate for the Heathrow option.

The true strength of opposition to the third runway emerged last week after the Department for Transport revealed details of responses to its consultation document. Out of nearly 70,000 comments, just 11% supported expansion.

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