In the past few minutes, the Airports Commission has published its interim report. It's the first big opportunity for there to be a political row over aviation capacity since the government kicked the issue into the long grass, and Sir Howard Davies' shortlist in this interim report certainly provides some opportunities for a row. The shortlist as it stands does not include Boris Johnson's favoured Estuary Airport, instead favouring new runways at Heathrow and Gatwick. The Commission will now consider a 3,000 metre runway south of the existing runway at Gatwick, and two different runways at Heathrow: a 3,500 metre runway to the northwest of the airport, and a 6,000 metre runway to the west which could be run as two separate runways.
Davies has, for the time being at least, dismissed Boris Island because it 'would be extremely expensive', 'would present major environmental issues' and require a 'substantial' level of new infrastructure. But Boris' aides are insisting that this is still a victory for the Mayor because the idea has only not made the shortlist, rather than being ruled out.
So what now? Well, Zac Goldsmith has already made perfectly clear what it is that he will do if the government supports expansion of Heathrow. The Mayor will react shortly too. And this is only the interim report. The Commission will only publish its final report in summer 2015, after the General Election, so the parties don't need to plump for any option until then. How easy that will be in practice is another question.
But the political problem is that before the last General Election, David Cameron made his 'no ifs, no buts' pledge on an additional runway at Heathrow. The Prime Minister has always taken great care to avoid a tuition fee-style U-turn on a promise, wary of the damage it can cause a leader and their party. He has been particularly stubborn on issues such as the winter fuel payment. Will he behave in the same way over Heathrow?