It's the ferocity of Conservative MP Tim Yeo's attack on David Cameron, rather than the subject in question (a third runway at Heathrow) , that is most notable. "[T]he Prime Minister must ask himself whether he is man or mouse," the former environment minister writes, before damning Cameron with the faintest of praise "as the leader who made the Tories (nearly) electable again". He goes on to compare him unfavourably  to Harold Macmillan ("presiding over a dignified slide towards insignificance") and finishes with the requisite reference to Thatcher (a Tory leader who won elections).

The reason Yeo's intervention is damaging for Cameron is that the chair of the energy and climate change select committee, who cannot be dismissed as a rent-a-quote maverick, has vocalised the concerns held about his leadership across the Conservative backbenches. Tory MPs increasingly fear that Cameron, to borrow Thatcher's phrase, is not "one of us". The Prime Minister's heart, writes Yeo, is "an organ that still remains impenetrable to most Britons".

For Cameron's MPs, his willingness (or not) to abandon his opposition to a third runway at Heathrow has become a litmus test of whether he is a true Tory. But even for the PM, a man with a penchant for U-turns, this would surely be one policy reversal too many. Both the Conservative manifesto and the Coalition Agreement explicitly opposed a third runway and the presence of Liberal Democrats in Cameron's cabinet (a political reality many Tory MPs conveniently ignore) means that the PM would struggle to change course even if he wanted to. With Ed Miliband opposed to a third runway on principle (he almost resigned as Climate Change Secretary over Gordon Brown's support for the proposal), Cameron will also face no pressure from Labour to change course.

Yet Justine Greening's faltering performance on this morning's Today programme suggests that the Transport Secretary has little confidence in the PM's word. Repeatedly asked whether she could remain in the cabinet if the government backed a third runway, she initially ignored the question (amusingly, she declared: "Yes, I did do a campaign against a third runway. But really this is not a full length runway") before finally conceding: "It would be difficult for me to do that". At no point did she state that she would not be forced to resign because the policy is not changing. When she declared her interest ("my constituency is under the flightpath"), before swiftly adding, "so is Philip Hammond's, as a matter of fact, my predecessor", she sounded like a woman desperate to avoid being reshuffled.

But it's election leaflets such as the one above, issued by Greening, that mean the odds are still against such a flagrant breach of trust. For the aviation minded, "Boris island" or a new hub airport, as proposed by Lib Dem MP Julian Huppert, still looks a better bet.