Architect Norman Foster says a Thames estuary hub is essential for Britain's economy; critics warn of a £50bn white elephant that could harm the environment

What is at stake, according to all sides of the argument, is nothing less than the economic and spiritual future of the nation. We are in danger of "denying future generations to come", says architect Lord Foster. It is about the importance of our "world-class natural environment", says the Royal Society for the Protection of Birds. It could be a "white elephant" that would deal a near-fatal blow to our economy, says Sir Terry Farrell, another leading architect. Also at stake is national identity: how much Britain should try to match growing countries such as China, and how much we should do our own thing.

They are talking about airports, more particularly the idea of the "hub", the place where airlines choose to have interchanging flights, which is not only good for the airport business but also any business that relies on the best possible air connections. Heathrow is such an airport now, but its two runways are at 99% of their capacity, and air travel keeps growing, so it is in danger of losing ground to Frankfurt, Amsterdam and Paris. A third runway, deeply unpopular with people living under its flight path, has been ruled out by the government, the opposition, and the mayor of London.

So last week Foster, in partnership with engineers Halcrow and economic consultancy Volterra, unveiled a plan for Thames Hub, a four-runway airport to be built on the Isle of Grain in north Kent, on the Thames estuary.

Building anew would achieve the best possible integration of planes and trains, the best provision for logistics, and the most modern, efficient terminals. Planes would mostly approach over water rather than densely populated areas. It would connect to the high-speed rail link built for the Channel tunnel and provide tens of thousands of jobs for the never-quite-achieved revitalisation of the area known as the Thames Gateway.

It is not the first plan to build an airport in the estuary. An attempt to build one at Foulness in Essex was scuppered by the 1973 oil crisis, and more recently Cliffe, near the Isle of Grain, has been mooted, but the Foster plan is the most ambitious. It is not just for an airport, but a new tidal barrier to protect London from flooding, a high-speed orbital railway that would roughly follow the path of the M25, and railway connections to seaports and northern cities. The total cost is put at £50bn, with benefits to the economy put at £150bn. Backers say that they are attracting interest from private investors.

Foster's inspiration is China. In the 1990s he designed Hong Kong's new airport, which required the levelling and reshaping of a bumpy island. He also designed the gigantic Terminal 3 in Beijing, which took four years to realise and opened in time for the 2008 Olympics. Now an even bigger airport is already being planned for the city. Foster has long admired the speed with which these were built, and laments how Britain has dithered about London's airports. Heathrow's Terminal Five took 26 years from conception to completion, including the longest planning inquiry in history.

Britain wasn't like this, says Foster, in the age of the great engineering projects. He urges that we "recapture the foresight and political courage of our 19th-century forebears", which means action to speed up and simplify the process of planning and public inquiries, and dealing less tenderly with the many objections projects like this provoke. He raises the spectre of Bric, the growing nations of Brazil, Russia, India and China. If Britain does not match their investment in infrastructure, "we are rolling over and saying we are no longer competitive – and this is a competitive world. So I do not believe we have a choice."

There are certainly objections. The Isle of Grain is not an abstract piece of nothing, but a rare wilderness surprisingly close to London. It is part of the atmospheric flat lands where Dickens set the opening of Great Expectations, and the airport would not so much be built on it as completely annihilate it. In the Thames estuary there are, says the RSPB, up to 200,000 birds, and another 30,000 in the nearby Medway, a population "of global importance" which is unlikely to mix well with an airport. Huw Thomas, a director of Foster & Partners, says replacement habitats could be created elsewhere, but the RSPB is unconvinced. Neither will it be easy to run high-speed trains through the green belt unopposed.

Farrell questions whether Foster's infrastructural wonderland would really work. The airport is "on the wrong side of London for growth – the heart of Britain is clearly on the other side". If Heathrow were shrunk or closed, he says, the investment that has gone into the airport would be squandered. More than that, the huge array of businesses that have grown up around Heathrow, from corporate headquarters in the Thames valley to hotels and warehouses and the UK's biggest food distribution centre, would have to relocate. Heathrow currently creates nearly 80,000 airport-related jobs, and many more in associated businesses. Homes for all these workers, with their schools, hospitals and shops, would have to be recreated in the east. No one planned that Heathrow would be what it is now, but for all its faults it is an extraordinary success, which should not be lightly discarded.

"Can we afford to flip London over?" Farrell asks, and cites Montreal-Mirabel airport, which opened in 1975 as the biggest in the world, misjudged its market and ceased passenger flights in 2004. Its main problems were its distance from the city and the introduction of longer-range aircraft, making them less likely to stop over in Montreal. The Foster plan carries some of the same risks, such as having a less convenient location than the existing airport and requiring a long-term bet on patterns of flying that may change.

Farrell argues instead for "consolidation of what we've got", for better train connections between existing airports, for example, so that they can work better together. "Foster is right to propose his hub," he says, as a contribution to debate, but we should not be dazzled into accepting it uncritically. Such solutions are "tremendously glamorous and sexy", but "you can't just take the say-so" of people such as architects and engineers, with a vested interest. Nor that of the mayor of London, Boris Johnson, an enthusiast for estuary airport plans, given that relocation would shift the environmental problems "from thousands of his voters, and dump them on someone else's".

Farrell argues that what works in China may not work here: "They have a growth economy and can afford to make mistakes." And China, not being a democracy, doesn't have to worry too much about opposing voices. "We can't emulate the Chinese. We've got to find our own position, which could be very clever and very smart, but different."

This debate assumes that endless growth in air traffic is desirable and inevitable, although it contributes significantly to climate change. It also enjoys the remarkable tax break of exemption from VAT on fuel. Should this ever end, people will fly less.

Meanwhile, engines are becoming quieter, which alters the discussions about noise pollution, and with the Airbus A380 aircraft are becoming bigger. John Stewart of HACAN Clear Skies, which campaigns to control the effects of aviation over London, thinks Heathrow could expand by handling larger planes for long-range flights, while high-speed trains would take over much of the short-range traffic. If he is right, it may not be necessary to build a new super-hub.

What is most striking is that no one knows for sure which option is best. This may be the most critical decision on infrastructure, environment and planning that this country has to take, but the implications and complexities are too big for anyone to have mastered them yet. The Foster hub could be as successful as Hong Kong, or a new Montreal-Mirabel. Confident though they are, the Foster camp acknowledge that their hub is partly speculative. Farrell isn't saying for sure that his idea of consolidation is the best one, but only that it deserves full investigation.

Whether either, or something else, is the best option is for the moment almost pure guesswork.

Rowan Moore is architecture critic of the Observer

Conran retrospective, New Review page 36