The Government is getting "increasingly interested" in the idea of a new airport in the Thames Estuary, London mayor Boris Johnson said today.
Mr Johnson already firmly backs a new "Boris Island" airport and architect Lord Foster has produced plans for a £50 billion airport on the Isle of Grain in Kent.
With a third runway ruled out at Heathrow, the Government is set to include the Thames Estuary option in a consultation on UK aviation this spring.
Prime Minister David Cameron is said to be growing keener on a Thames Estuary airport plan.
On BBC Radio 4's Today programme today, Mr Johnson said: "I think that where we are is that the Government is increasingly interested in this idea.
"I genuinely believe that they see not just the overwhelming aviation argument and the argument from international competitiveness in making sure that Britain has a hub airport that is viable for the long-term future. I also think the Government understands the massive regeneration potential of this project."
He added: "You can't go on expecting Britain to compete with France and Germany and other European countries when we simply can't supply the flights to these growth destinations - China, Latin America. We are now being left badly behind."
But Mick Rix, the GMB union's civil aviation industry national officer, said a Thames Estuary plan was "plain daft".
He said: "GMB consider that this is profoundly wrong and calls on all political parties to reopen the issue of the third runway at Heathrow."
A Department for Transport spokesman said: "No decisions have been taken. As the Chancellor made clear in his autumn statement, we will explore all the options for maintaining the UK's aviation hub status with the exception of a third runway at Heathrow.
"The Government will consult on a sustainable framework for UK aviation this spring, at which time we will set out our long-term plans for the sector."
Colin Matthews, chief executive of airport operator BAA, which runs Heathrow, told the Today programme: "An island airport is very long and very expensive. Even if it is agreed, it will be decades away. We need jobs and we need growth in this economy today."
He added: "You can't have two hubs. You can look at various cities around the world who have tried to do that.
"It is either Heathrow or it is another, and the consequences of closing Heathrow wouldn't just be big for my company, it would be big for 100,000 jobs in this part of London. It is a huge issue economically, it is a huge issue politically."
Having scrapped Labour's plans for a third Heathrow runway, the coalition Government had, originally, ruled out expansion at any south east England airport.
Chancellor George Osborne's Autumn Statement, however, indicated that only the Heathrow option was off the table.
Lord Foster's ambitious plan envisages a 24-hour, four-runways airport, with each runway 2.5 miles long and with facilities to cope with 150 million passengers a year.
Such a scheme would certainly meet the demands from airlines and big business for more airport capacity so the UK can compete with European rivals.
Last week, the calls for expansion were echoed by the UK's Civil Aviation Authority (CAA).
"Additional capacity would offer significant benefits for consumers and for the UK as a whole, so long as it is delivered in an environmentally sustainable way," said CAA chief executive Andrew Haines.
He went on: "However, as we haven't built a single runway in the south east of England capable of handling Boeing 747s and Airbus A380s for over 70 years, the difficulty of increasing capacity is obvious.
"The challenge facing the Government is to create an aviation policy that stands the test of time - not a policy for five years, but one for 30 years.
"If the private sector is to have sufficient confidence to deliver additional capacity then it needs to be convinced that Government policy is based on robust evidence and is likely to last for at least a generation."
Friends of the Earth's executive director, Andy Atkins, said: "A new airport in the Thames estuary would have a devastating impact on local communities and the environment - and all for pie-in-the sky economics that simply don't add up.
"London doesn't need another hub airport - the capital already has more flights to the world's main business destinations than our European neighbours.
"David Cameron's pledge to lead the greenest Government ever will ring hollow if he gives the green light to a huge expansion in air travel."
Simon Buck, chief executive of the British Air Transport Association, said: "The Government needs to maintain and build UK connectivity to emerging markets through permitting privately- funded additional extra airport capacity where it is most needed and demand is greatest - in the south east of England.
"Without this, the competitiveness of the UK economy will continue to be eroded and jobs will be lost."
Colin Stanbridge, chief executive of the London Chamber of Commerce and Industry, said: "The consultation is recognition that there is a desperate need for more airport capacity in the South East.
"Any new airport will not become operational for decades. yet the extra capacity will be required far sooner.
"It is therefore important that solutions which can be put into place much faster, such as the third runway at Heathrow, are not ruled out to ensure that London doesn't fall further behind other international centres."
John Stewart, chairman of the Hacan organisation, which represents residents under the Heathrow flight paths, said: "There are mixed views in west London about an estuary airport.
"Some residents support it as it would remove the constant noise from their areas. Others are concerned that job losses would blight west London as Heathrow directly employs over 76,000 people."
He went on: "The bigger question is whether extra capacity is actually needed.
"Even at Heathrow, nearly a quarter of flights are short-haul. If policies were in place to enable these passengers to switch to rail or do their business via video-conferencing, that would free up capacity for additional long-haul flights from the developing economies of Asia, Africa and South America.
"The question is not whether new capacity required but whether we are making the most intelligent use of existing capacity."
Downing Street said that the Government had made clear in last November's Autumn Statement that it was ready to "explore all the options for maintaining the UK's aviation hub status, with the exception of a third runway at Heathrow".
A spokeswoman added: "We will be consulting on the sustainable framework for UK aviation this spring."
Protect Kent welcomed the public consultation into the proposal for airports in the Thames Estuary.
The group, which promotes the diversity of rural England, has argued that the airports would have a disastrous impact on land both north and south of the Thames.
Andrew Ogden, campaigns manager for Protect Kent, said: "While our views on this consultation may appear to contradict our normal stance, we anticipate that this opportunity to share all of the facts and figures behind these proposals will expose them as the futile schemes they are.
"Over the past 60 years there have been many ventures to build airports in and around the Thames Estuary but none have ever passed the planning stage.
"Together with other campaigning and environmental groups, we will be presenting our case against these airports in response to the consultation.
"We will also be calling for a referendum to be held among all residents likely to be impacted by these proposals, both sides of the estuary."
Sir Richard Branson's airline, Virgin Atlantic, said: "In a week when the Chancellor has been championing the need for more investment from, and business with, the Far East, it is welcome to see the Government finally admit that the current capacity problem in south east England cannot be allowed to continue.
"The UK is becoming hamstrung by our lack of opportunity for growth and risks losing valuable business and tourism to our European competitors.
"We look forward to being a full participant in the consultation.
"However, we believe the Government should consider all the possible options, not just a Thames Estuary airport. The UK needs one strong and viable hub, and the impact on jobs and business of moving this away from west London must be considered."
Oliver Richardson, the Unite union's acting national officer for civil aviation, said: "We welcome that the Government has woken up to the critical importance of aviation to the UK economy and the significance of a hub airport.
"We would be highly critical of any consultation which only looked at Boris Island. Over 100,000 people are currently employed at Heathrow. A vanity project like this would wreck the economy of west London and blight the world reputation of London.
"The UK is falling behind its international competitors.
"As the UK's biggest transport union, we recognise that there needs to be a proper and informed debate about the modernisation of Britain's transport links, but Boris Island is a non-starter."
Leaders on Medway Council have called on Transport Secretary Justine Greening to attend an urgent meeting so she can hear their objections to the scheme.
In an open letter from the four group leaders on the authority, they said that 76% of the UK public are opposed to the airport proposal, along with many major airline industry leaders.
It said: "If it were to go ahead, it would have a huge affect on the lives of hundreds of thousands of residents in Medway, as well as across Kent and the wider Thames estuary, and would devastate an area of global environmental significance providing a home for around 250,000 migrating wildfowl annually.
"An airport would cost up to £70 billion, would require huge highways and infrastructure and would cut great swathes off the green belt and countryside.
"We strongly urge you to keep to Government policy and continue looking at fully utilising the capacity of existing airports - such as the five London already has (which is two more than New York) and others such as Manston and Birmingham, which could both be joined to London by high speed rail.
"We look forward to meeting with you at the earliest available opportunity to discuss this urgent matter further."
Foster + Partners, Halcrow and Volterra - the team behind proposals for the Thames Hub, which includes a new estuary airport - welcomed news about the aviation consultation.
Speaking on behalf of the team, Huw Thomas of Foster + Partners, said: "We are committed to working with the Government and wider industry stakeholders to put in place the transport connections Britain needs to encourage growth, job creation and trade with the rest of the world, particularly the emerging economies.
"This is an opportunity to reassert Britain's role as a global hub and an international gateway."