The Sunday Telegraph reports that “David Cameron has decided it would be politically safe to back a third runway at Heathrow, despite previously promising to block the expansion of Britain’s busiest airport,” and that “the Government is preparing to announce the next phase for airport expansion within weeks”.

If true, that would at first glance seem to signal a by-election in Richmond Park, since Zac Goldsmith has promised to stand down if growth at Heathrow gets the green light.  He apparently believes a decision to quit his seat after the Mayoral election result rather than before would be a breach of his pledge – although such timing would at least leave him clearer about his fate.

In the event of an announcement soon and a Goldsmith resignation soon after, the Conservative candidate for the London Mayoralty would oppose Government policy, which would do his prospects no harm, and the by-election could well be lost, which would do the Prime Minister no good at all.  The paper claims that George Osborne is keen on a quick decision.

However, the Telegraph also says that Cameron “is understood to want all the analysis to be completed before he comes to a verdict”.  Maybe the decision will be staggered in some way – or delayed yet – allowing the crucial announcement to be made when the Mayoral poll is safely done and dusted.