Heathrow is full. Those three little words send sober people into paroxysms. The worried wealthy of Holland Park, Chiswick and Kensington, realising that they may be under the flight path of the proposed third runway, are thronging to protest meetings. Green groups slam the Government's consultation, soon to end, as a sham. The four mayoral candidates for London have taken out full-page adverts declaring their opposition. Business leaders outdo Greenpeace for apocalyptic language, claiming that the airport is “critically important to economic prosperity” and making dire predictions that firms will flee abroad. “Heathrow can either decline or develop,” says Future Heathrow, a business lobby group. “It cannot stay as it is.”

That this last statement is manifestly untrue has not stopped the Government succumbing to the hysteria. Some of Ruth Kelly's Cabinet colleagues are incredulous that the air industry is being licensed to undermine other policies. Like a woolly mammoth that missed the Ice Age, the Department for Transport trundles on with its goal of doubling UK flights in 25 years, despite the many authoritative reports that show that this will make it impossible for Whitehall to meet its climate change commitments.

Forget joined-up government: while the Environment Secretary urges retailers to phase out old-fashioned light bulbs, and the Treasury dubs air travel so evil that passengers must pay more tax, Ms Kelly blithely waves forward flights, runways and the roads that connect them.

There has always been one rule for the air industry and another for the rest. While the Treasury defends petrol taxes partly on environmental grounds, the airlines continue to pay no tax on fuel. The EU is imposing standards on car manufacturers that have them screaming, but it cannot touch the air lobby. The DfT dropped its “predict and provide” approach for cars years ago, recognising that the creation of roads led inexorably to more traffic and that environmental considerations made rationing essential. But it continues to predict and provide for the air industry, refusing to consider that demand should be curtailed.

The double standard has led to a rash of broken promises on Heathrow. Terminal 4 was approved in 1978, subject to a cap on annual traffic movements of 275,000. Two years later BAA recorded 287,000 movements, and 376,000 in 1990. When Terminal 5 was approved in 2001, the planning inspector and BAA stated that a third runway would be “totally unacceptable”, and set a new cap of 480,000 movements. But by 2003 a White Paper aimed at 700,000.

The justification is the importance of aviation to the economy. It would be foolish to argue that air travel is not important to business. But some of the mythology is misleading. The growth in air traffic is overwhelmingly from leisure travel, not business. More than 80 per cent of international travellers at UK airports, and 60 per cent at Heathrow, are holidaymakers. Outbound tourism outstrips inbound, creating a whopping £18 billion balance of payments deficit. Only this week, the Travelodge chain of hotels called for an end to unfair tax breaks for budget airlines, which it said were “the single biggest cause of decline in traditional [UK] tourism resorts”.

It is one thing to treat the air industry as a special case, it is quite another thing to distort the facts. And here the Government's collusion with the industry is a problem.

Take the greenwash first. Ministers repeat the mantra that “Heathrow's expansion will only go ahead within strict environmental limits”. But they know full well that the absence of legal standards on noise leaves communities defenceless. New EU air-quality standards had looked like an insuperable hurdle to a third runway, but are being fudged with ropey claims that road traffic emissions will fall. Two weeks ago the Advertising Standards Authority ordered British Airways to withdraw the claim, made by its CEO in an e-mail to Executive Club members, that the third runway would reduce carbon dioxide emissions because aircraft would no longer have to waste fuel queueing to take off or land. This flatly contradicted Whitehall models, which assume that the new runway will raise CO2 emissions by 2.6 million tonnes a year from the 200,000 extra flights.

Secondly, take the arguments about capacity. BAA's figures demonstrate clearly that Heathrow is not full. Not remotely. The appendix to the government consultation on the third runway states that 67 million passengers used Heathrow in 2006, and that this could rise to 122 million if a third runway were built. But it also shows that 95 million people could use Heathrow if “maximum use were made of existing runways”.

At one stroke we are looking at a con, perhaps the greatest ever perpetrated on the British people by the DfT. For BAA itself is telling us that 28 million more people could use Heathrow without a new runway and without breaking the cap on flights.

How? By using larger planes and filling more seats. Jeff Gazzard, of Airport Watch, says that if Heathrow were not allowed to expand, it could spur the airline industry to invest more rapidly in larger aircraft like the A380, on which some are already hedging their bets.

Bigger planes would not solve climate change, although they would reduce local pollution. The point is that we have been told that Heathrow is full when it is not. That kind of distortion suggests that the DfT has ceased to function as an arm of government and has become a mere subsidiary of BAA.

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