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Oh dear. It was not supposed to be like this. Both Conservative and Labour politicians had hoped that the controversial question of expanding airport capacity in the south of England had been delayed until after the election. That is why the Airports Commission was asked to report after May 2015.

The Conservatives, in particular, had hoped to go to the polls with a constructively ambiguous manifesto saying they supported the commission but would not prejudge its conclusions.

But Sir Howard Davies' interim report leans so far towards expanding London Heathrow that it will make harder for the parties to maintain their collective omerta for the next 18 months. This is why:

1. Opponents of Heathrow expansion, whether Boris Johnson, Zac Goldsmith or other residents on the ground, will be more vocal now they have a target. They will not let this matter rest. One local Labour MP, John McDonnell, talks of the "biggest, greenest, direct action campaign ever". Mr Goldsmith threatens by-elections. Opponents believe that expanding Heathrow is politically impossible and will do their utmost to make that case. In the face of such an onslaught, it will be hard for the party leaderships to stay silent.

2. Some in Labour will feel tempted to come out in favour of Heathrow expansion to display their pro-growth credentials at a time when the party is searching for economic credibility. The fact that this approach might also discomfit and divide the Tories would be a Brucey bonus. Ed Miliband is still said to be "sceptical" about Heathrow expansion but that is often said by Labour insiders with a knowing smile, and publicly frontbenchers talk of trusting the Davies commission and its work.

3. Some Conservative ministers might want to cut their losses and express their private support for Heathrow expansion publicly. Can the Tories, they argue, really go into the next election as the pro-growth party without saying what they would do on one of the biggest long term investment issues this country faces? George Osborne, we are told, is much more supportive of Heathrow expansion than he once was. And, these ministers point out, if you want to drive airport expansion through, it would be better to have put it to the electorate first so you can claim some kind of mandate when the going gets rough.

4. Other Conservatives argue that the party effectively carried out its U-turn on Heathrow when it set up the Davies commission without repeating their manifesto commitment to rule out a third runway. And if you have sold the pass, they argue, if you are facing accusations of betrayal, you might as well be upfront and make the case. They say that either way, by supporting the Davies process, the Tories will have to go into the election with a manifesto that no longer rules out Heathrow expansion. So why not take the hit now rather than closer to the next election?

As for Boris Johnson, his favoured solution of a hub in the Thames estuary east of London lives to fight another day - just. As he said, it is "not dead yet". So the potential for all out war between the London mayor and the Conservative Party is put on hold until next year. Mr Johnson can now spend the next few months making the case for east London rather than opposing Heathrow. But Sir Howard Davies says he will decide finally on whether the east London option will make it onto the formal shortlist next year. And if he rules against it, then the potential for fireworks between City Hall and Downing Street is huge.

So today both the Tories and Labour will welcome Sir Howard Davies' interim conclusions but they will be carefully non-committal about what they plan to do. The question now is how long that position remains tenable.