Despite the formidable array of political and social forces arrayed against the Government when it took its controversial decision to give the green light to a third runway at Heathrow airport, ministers could always cling to the unswerving support of the business community for the project.
But now it seems even that pillar is crumbling. A group of influential business leaders - including the chief executive of Sainsbury, Justin King, and Charles Dunstone of Carphone Warehouse - have called on the Government to reconsider its plan to permit the world's busiest airport to expand.
According to the group's organiser, Ian Cheshire, chief executive of the home improvement group Kingfisher, there are many others in the business world who feel similarly unconvinced of the economic merits of the third runway. The group regards itself as the vanguard of a new wave of resistance.
They have certainly entered the fray with some robust arguments. The aviation lobby asserts that a third runway would enable Heathrow to serve a wider range of destinations, thus opening up Britain more fully to the world. But this alliance points out that the primary effect of building Terminal Five at Heathrow has been to increase the number of flights on already popular routes. There is no reason to believe that the consequences of a new runway would be substantially different.
The group also punctures the argument of the airlines that London will lose business to European competitors if the runway is not built, pointing out that the capital is served not only by Heathrow, but Gatwick and Stansted too. London already has adequate runway capacity. The transport infrastructure the capital, and indeed the country, really needs is high-speed inter-city rail, as the Conservatives have been pointing out.
The anti-third runway case would seem to be supported by yesterday's results from BAA, Heathrow's owner, showing that passenger numbers fell by 10 per cent in the first three months of this year. What sense does it make to acquiesce to those lobbying for airport expansion, when demand for flights appears to be contracting? The airline industry will counter that this fall is a consequence of the global recession and that when the economy picks up, so will the fortunes of the airline industry. No doubt, there will be some bounce back in passenger numbers as economic confidence returns. But there is another constraint on passenger numbers looming that the airlines will find hard to dismiss.
There are indications of a new mood of urgency among world leaders - most significantly on the part of the United States President Barack Obama - with regard to the threat posed by rising greenhouse gas emissions. And aviation is the fastest growing contributor to these climate-altering emissions. For a global agreement on reducing carbon emissions to be credible it will have to include measures to eliminate the many hidden subsidies the aviation industry has enjoyed over the decades, from untaxed fuel to special bankruptcy protection.
The business models of the airline industry have long been based on the presumption of ever expanding demand for flights and the perpetual indulgence of national governments. Both those presumptions are looking increasingly shaky. Like Britain's ministers, the aviation sector needs to wake up to some stark new economic and environmental realities.