Nicholas Faith: Boris Island must never be able to get off the ground

At first the idea of a new airport on, or even in, the Thames Estuary seemed to be just one of the Mayor of London's less amusing flights of fancy, but now the arrival of a proposal by the architect Norman Foster has given it some respectability. The idea of replacing Heathrow and moving east is not new. Forty years ago, Maplin Sands, off Essex, was held up as a possible site, but interest soon dwindled and the present proposals seem just as likely to fade when confronted by the real situation: that a new airport is both impractical and unnecessary.

The cost would, of course, be staggering. Lord Foster – who has designed three splendid terminals, at Stansted, Beijing and on an island off Hong Kong – reckons the total cost of his project would be around £50bn, which would include a new London orbital railway. Rather airily, he assumes the money could be raised internationally. In reality, any new major airport anywhere near the Thames is doubly impractical because of politics and birds.

The idea of replacing Heathrow, which employs 75,000 people, as an international hub and moving it across London boggles the mind, affecting as it would a dozen or more – mostly marginal – constituencies. But the existence of 300,000 permanent resident birds on the banks of the estuary is decisive in itself. They now occupy five Special Protection Areas which makes Lord Foster's claim – that they could be replaced by a man-made bird sanctuary – ridiculous if only because, as the RSPB puts it, "they'd keep coming back".

Even more decisively, the Civil Aviation Authority has a Bird Hazard Management Plan which requires a bird-free zone around any major airport. Low-flying aircraft are particularly susceptible to bird strikes, a hazard simply impossible to control (the miraculous emergency landing of an airliner piloted by Captain Chesley Sullenberger on New York's Hudson River was caused by a flock of birds disabling both its engines).

Both new airport proposals – Boris Johnson wants to create a new island ("Boris Island"), Foster would build his on the Isle of Grain on the estuary's south bank – assume Heathrow is crucial, not only as a final destination, but also for transferring passengers. In reality, over the past 10 years, the number of transit passengers at Heathrow has slumped from 341,000 to a mere 136,000, a tiny fraction of its total of 65 million.

Another delusion is that we need a Very Major Airport to demonstrate that we are a Very Major Player on the world business scene. John Cridland of the CBI gave the game away when he declared that "Britain will be left behind in the premier league of nations if ministers fail to increase runway capacity in or around London". In fact, of course, all we need is the ability for Londoners to take a plane to anywhere in the world. Moreover, once they get over a certain size, airports become decidedly inconvenient for passengers. At Schipol, Amsterdam's rival to Heathrow, many planes land virtually on the North Sea and must taxi for half an hour to get to a massive terminal which itself takes half an hour to walk through.

The case also ignores the fact that London is already served by five airports, two more than New York, for instance, and that the 20 outside South-east England already take millions of passengers from London.

In the last decade, while annual passenger numbers from London's airports have increased by around a fifth to 120 million – mostly at Stansted – those from England's 11 major regional airports have nearly doubled to reach more than 40 million. In some cases the increase has been larger: Liverpool's John Lennon airport has nearly tripled its numbers, and eight airports, including such unlikely ones as Southampton, now handle more than a million passengers each.

And these figures refer exclusively to those on scheduled services to Europe, where these airports take an ever-increasing proportion of long-haul passengers away from Heathrow to foreign alternatives like Paris and Amsterdam. The result of this drift from the capital is that in the past 10 years, the proportion of all UK air traffic using the "London Five" has declined by 10 per cent to little more than a half.

Cridland's declaration was in reply to the announcement by Birmingham Airport that, through a combination of runway extension and terminal construction, it could soon handle nine million more passengers a year – today it has less than seven million. It also has planning permission for expansion to well over 25 million. This would put it in the same league as Gatwick, as well as being able to handle the biggest aircraft on the longest routes from a base which could attract passengers from anywhere between Birmingham and the capital.

But the key to accommodating any increased traffic is not only encouraging expansion outside the capital, it also lies in dividing London's air traffic more sensibly between its five airports. Willie Walsh, now in charge of Air Iberia as well as British Airways, made this clear when he said he was buying the small, loss-making airline BMI for its numerous slots at Heathrow. These will be used for long-haul, rather than existing short-haul services. Basically he was saying there are still lots of short-haul flights from Heathrow – the average plane transports a mere 147 passengers, a number virtually unchanged in 10 years, demonstrating just how many flights are short-haul. The new slots could complete Heathrow's coverage of the globe, which now excludes much of Latin America and inland China. So Heathrow for long-haul, the other four for short-haul.

Of course any attempt to shuffle airports and destinations would be difficult, but could be helped by changing the basis for charging the fees paid by airlines, at Heathrow for instance, to discourage smaller aircraft by charging per aircraft rather than per passenger.

But the biggest opportunity lies in using Gatwick more efficiently, above all as an alternative to Heathrow for long-haul passengers. At the moment, a fifth of its services are by charter flights which could go to Stansted or Luton. This would allow more long-haul services – at present it has relatively few, virtually all to tourist destinations, without any to such major cities as Chicago, Los Angeles or Boston.

I suspect Gatwick's major problem is its inaccessibility by road from central London. This matters because the rich and self-important won't use trains to travel to and from airports even though there are separate, frequent and reliable rail services from Gatwick to the West End and the City taking a mere half an hour – far quicker than the journey by limo from Heathrow. The train could so easily take the strain from our airports and its passengers.