In the next few weeks – most likely on the eve of the Christmas recess - the Prime Minister will officially announce his decision on Heathrow expansion. This week the airport plans are back in the news amid reports of a Cabinet split over the government's policy and mounting opposition from Labour backbenchers. The Miliband brothers, Harriet Harman, Hilary Benn and John Denham are all said to be opposed to the runway plans, providing a glimpse of hope that the government might still rethink what they'd previously sought to present as a fait accompli. I gather that the removal of the 'Hutton-Kelly axis' opened up the question, 'Does anybody actually still want a third runway at Heathrow?'

It's surprising that it's taken this long for any serious Labour dissent over this to become apparent, especially when, to quote Jackie Ashley, "a swath of Labour ministers and MPs can expect to lose their seats if Heathrow's third runway is given the go-ahead." West London MP, Andy Slaughter, put it most bluntly when he told The Guardian, "This is a vote- changing issue for the electorate and with the Tories and the Liberals Democrats opposed to expansion, marginal seats will be lost unless the policy is rethought."

The momentum behind the Heathrow campaign is incredible but what has been particularly memorable and powerful about this movement are the alliances that have formed between radicals and Middle Englanders. The day after activists from Plane Stupid climbed onto the roof of the Palace of Westminster to highlight the anti-democratic nature of the so-called 'consultation' on expansion, The Sun captured this mood with its lead editorial. "Beneath the antics on the Commons roof is a serious issue that matters to millions. Airport noise and pollution blight many lives. Expanding Heathrow may benefit the economy — but at what price to the environment?"

That other Conservative-leaning papers including The Sunday Times, The Evening Standard and The Daily Mail have all spoken up over BAA's plans reflects the shift in public opinion and the space that's been opened up - only to be seized upon by Cameron's Conservatives. The Tories, like the Lib Dems, are now sufficiently convinced they're on the right side of the argument with Heathrow that they've gone so far as to say that if Labour approves the expansion and then they're elected, they'll cancel the contracts.

Cameron knows he's on safe ground. The coalition to stop Heathrow is perhaps the broadest in the UK since the Iraq war movement. It ranges from the National Trust and WI on the one hand, through to unions like UNISON and the RMT, residents groups and local authorities representing almost four million Londoners, and increasingly the emboldened grassroots direct action movement on climate change. Armed with peer reviewed scientific papers, the Heathrow movement has won the support of both of the government's own green watchdogs - the Environment Agency and the Sustainable Development Commission, as well as Labour's favourite think tank, IPPR.

Increasingly the movement is winning the economic arguments too. This is epitomized by the fact that both The Economist magazine and the former chief of British Airways say the economic case for expansion is flawed. The respected economic consultancy, CE Delft, says that the aviation industry's economic claims are totally overinflated and you don't have to be Paul Krugman to see that the government's position of trying to create the conditions for an artificial demand for flying simply won't be sustainable in the middle of both an economic and a climate crunch.

Ministers still have a few weeks to do the right thing but it's fair to say that we campaigners aren't holding our breath - even if the only people who still want new runways are BAA and their Spanish shareholders.