It looks as if the die is cast. A third runway at Heathrow is about to be backed by the government despite 70,000 public submissions against the expansion of London’s badly sited and oversized airport. Investigations by this newspaper have revealed that objections to the third runway have their basis in genuine concerns among experts about the impact on the environment and quality of life.

As we report today, the Greater London Authority is worried that the noise impact of Heathrow’s expansion has not been adequately taken into account and that the airport already exceeds pollution limits and will do so even more. It is highly sceptical of the claim by BAA, the airports operator, that new, quieter aircraft will reduce Heathrow’s impact.

Such concerns have been taken up by a few cabinet ministers who think Gordon Brown is ignoring the key drawbacks to Heathrow’s expansion or deliberately sweeping them under the carpet. They have been arguing privately that allowing the third runway to proceed is incompatible with the government’s attempts to portray itself as “green”. But while the cabinet minority may succeed in delaying the announcement, it appears that it will not prevent the go-ahead from happening.

Why is Mr Brown so hellbent on a third runway at Heathrow? Even if he does not favour this newspaper’s preferred alternative, a new airport in the Thames estuary, there are other options. Sir Rod Eddington, Mr Brown’s transport adviser, is known to believe that if London needs an international hub – and the economic benefits of this are hugely exaggerated – it should be at Gatwick, not Heathrow.

The prime minister’s determination to turn Heathrow into a bigger and even more unwieldy monster is partly macho political posturing. Only he, he thinks, has the vision to build an infrastructure “fit for the 21st century” while David Cameron plays around with high-speed rail links. Labour will have both a third runway and a rail link. Labour is taking the long-term pro-business line.

It is not clear, however, that there is a political or a business case for Heathrow’s expansion. Labour MPs representing constituencies around the airport fear the loss of their seats, a verdict backed up by polling. The assumption that air travel will grow as rapidly over the next 30 years as over the past three decades has to be questioned.

It is not too late for Mr Brown to change his mind. A cabinet go-ahead for the third runway will almost certainly result in a legal challenge. The Tories are opposed to it and should overturn the expansion if elected next year. Expanding Heathrow, when there are so many legitimate doubts and alternatives, looks more foolhardy than bold.