Final approach and initial take off from the site would be entirely over water, promising minimal noise disturbance for local communities. Controversial proposals for a £13bn third runway at Heathrow currently being considered by the British Government would involve demolishing an entire nearby community and carry a 'high risk' of breaching noise and air quality targets set by the European Union according to internal Department for Transport documents obtained under the Freedom of Information Act

Mayor Johnson cites Hong Kong's Chek Lap Kok airport, built on a levelled island and reclaimed land in the harbour, as an influence on his thinking. Chek Lap Kok was arguably the most significant project in the annals of dredging as it transformed projects planners' appreciation of the industry's capabilities and led directly to the massive fleet expansions and land reclamation projects currently underway in places such as Dubai. There is no doubt that Boris Island would be an equally massive dredging and marine construction project in the Thames Estuary.

However, the Government has already considered an airport proposal off the Isle of Sheppey and dismissed it as both too costly and fraught with environmental and safety issues, such as the threat to aircraft from high local levels of bird density.

The governing Labour Party was quick to attack the Conservative Mayor's proposal as a smokescreen to distract attention from his own party's perceived schisms over Heathrow expansion. That argument was destroyed the following week at the Conservative Party Conference in Birmingham, where the Tories announced that they would indeed scrap plans for a third runway at Heathrow.

'This is a seriously green decision', said shadow transport secretary Theresa Villiers. 'A few years ago it would have been inconceivable for the leader of the Conservative Party to say no to a third runway and putting the brakes on Heathrow expansion.'

Meanwhile, Kent County Council has come out firmly against the Boris Island proposal, preferring to support expansion of Kent International Airport at Manston, which already hosts daily Boeing 747 freight services and even without expansion has a current capacity to handle 700,000 passengers per year.