Mr Johnson and Mr Oakervee (right) visited proposed sites in January
There are no "insoluble" issues to prevent an airport being built in the Thames estuary, a report has concluded.
London Mayor Boris Johnson is considering building an airport several miles off the Essex and Kent coasts.
A feasibility study found no logistical constraint to the £40bn plan but called for further studies into its impact on the area's ecology and bird population.
The RSPB, Medway Council and Kent County Council jointly said the plan would "devastate the environment".
Mr Johnson has opposed the expansion of Heathrow airport but maintains that "further runway capacity" is vital and regards "Boris Island airport" - as the Thames estuary plan has become known - as a viable solution.
The study by engineer Douglas Oakervee, who helped construct a similar airport at Chek Lap Kok in Hong Kong, supported the plan.
"The area of the Thames Estuary does not present any insoluble issues regarding weather, geology, hydrography, shipping lanes, fishing activities, leisure services or other physical obstructions," Mr Oakervee said.
The airport would "accommodate possibly up to six runways depending upon demand," he added.
Mr Oakervee said the cost of the plan has been estimated at £40bn, including the construction of a high-speed rail-link connecting it to land and taking passengers through the area known as the Thames Gateway.
But he said integrated infrastructure could produce "substantial savings".
The report concluded that "very thorough and careful studies" on the airport's technical, environmental and economic benefits were required.
"This would particularly apply to those involved with the delicate ecology of the area and with expertise as to the very large bird populations," he said.
Further studies must begin by summer 2010 if the government wants to achieve further airport capacity by or before 2030, Mr Oakervee added.
A spokesman for Mr Johnson said: "The report concludes that, although there is no overwhelming logistical constraint to the construction of an airport, a holistic approach is needed when looking at any future use of the Thames Estuary."
He added: "The mayor accepts this recommendation, and is therefore setting up a Thames Estuary Steering Group... to oversee further study and research."
Medway Council, Kent County Council and the RSPB have jointly set up an online petition against the "undeliverable, unaffordable and unnecessary" plan.
Medway Council leader Rodney Chambers said: "The Thames Estuary airport would require huge unsightly highways to be built linking the airport to the motorway network, turning parts of Medway and Kent into a concrete jungle.
"The airport and the infrastructure needed to serve it would cost a ridiculous amount of money and it would devastate the environment which includes Sites of Special Scientific Interest and internationally important areas where hundreds of thousands of birds migrate to annually."
Conservative leader David Cameron and shadow transport spokeswoman Theresa Villiers have not supported plans for either the Thames estuary airport or Heathrow's expansion.
Labour's environment spokesman on the London Assembly, Murad Quershi, said the Thames airport would be "an expensive, logistical nightmare" with "the potential for dangerous bird strikes".
Institution of Civil Engineers director general Tom Foulkes said he welcomed the feasibility study's "holistic approach" to the Thames airport plan.
Mr Foulkes said: "This approach rightly considers energy, surface transport and flood defence, and not just the airport itself.
"We look forward to seeing further research into the project's feasibility and desirability."