Thames airport backed by Downing St

Plans for an airport in the Thames estuary have won political backing in Downing Street and the Treasury as a visionary way to solve Britain’s aviation capacity crunch.

Until recently the concept of a new “hub” airport in the estuary – which would supplant Heathrow as Britain’s largest airport – was seen as an eccentric option pursued only by London’s Tory mayor, Boris Johnson.

But in recent months George Osborne, the chancellor, has switched from outright scepticism and is now looking favourably on the idea, according to several sources. There is also support in Downing Street.

The estimated cost of a four-runway project on reclaimed land on the Isle of Grain, put forward recently by architect Norman Foster, would be £40bn-£50bn – a big sum at any time, let alone during the current squeeze on public finances.

But supporters believe that much of the financing could be obtained from the private sector or overseas government wealth funds.

An estuary airport would pose other challenges including the danger of bird strikes, the impact on wildlife and possible use of Dutch airspace for flight paths.

Senior Tory figures are open to either the Foster project or a previous incarnation of the concept, promoted by Mr Johnson, on a so-called “Boris Island” nearby.

Steve Hilton, Downing Street’s influential head of policy, has also emerged as a staunch advocate for the cause and has been seeking to persuade cabinet ministers of its potential.

Mr Hilton is understood to see the project as a latter-day equivalent of the Channel tunnel, a scheme which was seen as “lunatic” at the time but is now taken for granted by the public.

The replacement of Philip Hammond – the former transport secretary who did not back the Thames scheme – with Justine Greening is also seen as a positive by its advocates. Ms Greening, MP for Putney, campaigned strongly against the expansion of Heathrow.

All airports in the south-east are expected to be full to capacity by 2030, a fact that has caused immense frustration among business leaders.

The coalition has ruled out a third runway at Heathrow and second runways at Gatwick and Stansted, prompting an urgent quest to find alternative capacity.

Submissions have already been made to the transport department for an aviation review, which should produce an interim report in March next year and a final report in the spring of 2013.

“The concept of a Thames estuary airport forms a useful contribution to the debate and will be considered alongside all other responses,” the Department for Transport said.

Some government insiders had backed a novel “Heath-wick” scheme which would link Gatwick and Heathrow by high-speed rail – but the Treasury believes this has concept has not taken off.

The Foster airport would serve 150m passengers a year with planes approaching from the north-east over water rather than homes. The scheme involves a high-speed rail link and submerged tidal generators for energy.

Mr Johnson will launch a new policy paper calling for the government to take action to build more runways in a speech at the Institute of Directors on Monday.

The government is set to launch an infrastructure review later this month.