The engineer behind Hong Kong’s island airport has said London Mayor Boris Johnson’s plans to build an airport in the Thames Estuary “could be realised”.

After sailing out to the site with the Mayor last week, Douglas Oakevee said he saw “no overwhelming obstacle” to the scheme, which is being billed as an alternative to a third runway at Heathrow.

“The construction would be easier than other airports such as Chek Lap Kok in Hong Kong and Kansai in Japan,” Mr Oakervee said. “The ground conditions are much better."

The Tory Mayor urged politicians to take the plans seriously. So far, it has been opposed by not only environmentalists, but also his own party.

He would also have to raise as much as £40 billion from private investors to fund the project.

The Mayor, along with Mr Oakervee and Dr Graham Plant, a chartered civil engineer who has worked on building projects across the world, visited the site aboard a 122 metre dredger on Friday.

“The trip has reaffirmed in my mind that a new airport in the Thames estuary has got to be factored in as an option for London’s long-term aviation needs,” said Mr Johnson.

Doug Oakervee added: “I'm encouraged - from an engineering perspective it is quite clear that the Mayor’s ambitions could be realised.

“We have learned that there is a remote site where an airport could be built with minimal disruption to people of Essex and Kent.

“There are technical issues to be addressed but I see no overwhelming obstacle. We now need to work with other experts to address the challenges.”

Mr Oakervee is currently working on a feasibility study for the scheme and is due to publish his findings by the end of March.

Nick Raynsford, the Labour MP for Greenwich and Woolwich who chairs a cross-party parliamentary group on the proposal, also visited the site.

“I have always believed that the estuary was a potential site for London’s main airport and the visit has reinforced that view,” he said.

However, the Royal Society for the Protection of Birds (RSPB) has pointed out that the estuary is home to 200,000 birds in winter alone. Airport construction in the estuary “would be the most destructive development ever undertaken in the UK”, it has warned.

A 2003 proposal to build an airport alongside the Thames Estuary near the village of Cliffe in Kent was dismissed after 150,000 public complaints to the Government.

Jenny Jones said, a Green party London assembly member, warned that an estuary airport would pose “a serious risk of bird strike” such as the one that caused a plane to belly-flop on the Hudson River earlier this month.

"Planes and birds don't mix, as any resident of New York will testify,” she said.

"Boris is constantly putting blind optimism, ahead of the facts. He seems to think that just because he says there is no problem, it must be so.”