The wheels seem to be coming off the Heathrow expansion, if you’ll forgive the expression. The plans from the Department for Transport (DfT) for a third runway and more flights in the meantime don’t even convince the environment secretary, let alone the Environment Agency. The idea – if you can call it that – that we must facilitate an inexorable growth in air travel has taken a quite a hit lately.

Officially, the government is carrying out an extended consultation over proposals for a new runway and a sixth terminal at Heathrow from 2020, with extra flights from around 2012 through mixed mode (allowing both runways to be used at the same time for take-off and landing). Until recently, approval for expansion has seemed a foregone conclusion, mainly because the consultation was so obviously fixed in that direction.

But this week there have been reports that the cabinet is split over the runway and the Environment Agency has continued to express its opposition. As I reported yesterday on newstatesman.com, Chris Smith, a former Labour minister and chairman of the agency, has criticised the government’s attempt to delay new European air quality rules while increasing pollution from Heathrow. With an economic downturn and the government apparently deciding not to give aviation a free ride on carbon emissions, the prevailing wind may be blowing away from a bigger Heathrow.

The government’s case has always been that expansion will only go ahead if “strict environmental conditions” are met but the extent to which it has fiddled the figures, engaged in wishful thinking and moved goalposts makes its consultation the dodgiest official publication since the Iraq dossier.

Fortunately, unlike the dossier, we can see through the deception before the decision is taken. For example, we have seen how the DfT colluded with airport owner BAA to amend its modelling to give the right answers on air quality. Much of the DfT’s claim that pollution following a third runway will be within legal limits depends on disputed assumptions that planes and the cars on roads nearby will by then be so much cleaner that it won’t matter that there will be more of them.

Similarly, the Dft claimed in its consultation document:

“We believe that full mixed mode (540,000 ATMs) by 2015 would be compatible with compliance with the EU air quality limits for PM10, and NO2 in the vicinity of the airport without the need for further mitigation measures.”

If not an outright lie, this is tight-fisted in the extreme with the truth. The DfT was claiming here that in 2015 – the date by which mixed mode operation can be used to squeeze the maximum number of flights into two runways – pollution in the immediate area will be within the limits set out in the European Air Quality directive. Just as well, as the directive will definitely be in force by then, even if the government achieves a five-year delay.

What the DfT was reluctant to admit was that its own predictions show that the directive will be breached at Heathrow (and elsewhere) even before any expansion and that increasing flights from around 2012 would make things worse. If you get to “full mixed mode by 2015” by building to it up from 2012, then that isn’t “compatible with compliance with the limits”. The government’s case is that it won’t breach the limits because it will delay their implementation, but that isn’t the same as complying with them.

And neither the Environment Agency nor EU environment commissioner Stavros Dimas is convinced by the DfT’s claims that breaches of the directive after 2015 will magically disappear.

This kind of sleight of hand should make us worry that the government will fiddle the carbon figures too. In spite of dodgy claims from the aviation industry that the per kilometre carbon footprint of flying will soon be as low as a congestion charge-exempt car, even the government isn’t claiming that more flights to and from Heathrow won’t mean more carbon emissions.

Climate change campaigners have welcomed the government’s agreement in principle to include international aviation and shipping in the legally-binding UK carbon budget. Some see it as inevitably restricting aviation expansion while others worry that limits will be sidestepped by purchasing notional carbon reductions from other countries.

Will the economic downturn be the final nail in the coffin of Heathrow expansion? It’s tempting to think so, but the government is looking further ahead. It imagines that demand for air travel will rise over 10 or 20 years and worries that Britain won’t be competitive without a piece of it. But with demand falling in the short term, the sense of urgency around expanding aviation should abate. Perhaps the government won’t let the aviation industry and business bounce it into an early decision.

The government-sponsored Sustainable Development Commission has been arguing that decisions on expanding aviation shouldn’t be rushed, while there is so much dispute over the facts, never mind the policy. It said in September that the debate looks “immature”. Perhaps it had in mind a crass piece of triangulation from former Business Secretary John Hutton that: “we will help make flying greener rather than restricting people’s opportunities to fly altogether”.

If that’s the strength of the case for expanding Heathrow, no wonder people are increasingly seeing through it.