When did the Government decide to change its mind on airport expansion?

A parliamentary question answered by Aviation Minister Robert Goodwill on 23 November reminded DUP MP Jim Shannon of Prime Minister David Cameron's pledge that a final decision on airport expansion would be made by the end of 2015. Little over two weeks later, on 8 December, the Minister told Chichester MP Andrew Tyrie that a decision would be made in "due course".

Prevarication is the finest art form in politics, so what does it mean?

In practice, this means that we will be waiting until at least the summer of 2016 for a firm decision and perhaps even longer than that. Appearing on the Today programme the morning after the announcement of the delay, Transport Secretary Patrick McLoughlin made it clear that the Government would "hopefully" reach a verdict by then.

Headlines have inevitably focused on the indecision as a breach of David Cameron's pledge to reach a decision on Heathrow expansion by the end of the year. Media speculation viewed this very much as Heathrow's loss, given the briefings circulated by sources close to the process that a third runway would be given the go-ahead.

However, Mr McLoughlin has insisted that all three options, including the Heathrow Hub and a second runway Gatwick remain on the table. The loss of an early Christmas present for one has become a reprieve for the other two contenders. He was able to use the Davies Commission to partly cover this suggestion as its final report judged all three shortlisted options were viable.

Two reasons are held up to explain the Government's decision.

The first was explicitly acknowledged by Mr McLoughlin himself, specifically the need to tackle concerns around air quality and Heathrow expansion. He cited the Commons Environmental Audit Committee report as one of many factors ministers can use to justify further research before reaching a decision. Heathrow might even welcome the extra time to strength its green credentials in order to win more political support.

However, the second more complicated factor of the London mayoral elections presents the most imposing barrier to a third runway.

On 8 December, Conservative candidate Zac Goldsmith reminded journalists of his pledge to resign his Richmond Park seat if Heathrow expansion was approved, but added he would not resign as Mayor, if elected. Some reports say that he personally met with the Prime Minister shortly after.

Meanwhile, the Transport Secretary was at some pains to play this down, but that did not stop him from attempting to make political capital from what he called Labour mayoral candidate Sadiq Khan's inconsistency on the issue.

By deferring its decision, ministers have ensured that airport expansion will remain an intense, but academic argument between the two men most likely to replace Boris Johnson in City Hall. The longer the debate drags on, the more awkward it becomes for the Labour Party. Jeremy Corbyn and Shadow Chancellor John McDonnell are both opposed to Heathrow, but Shadow Transport Secretary Lilian Greenwood has a more ambiguous stance.

The Government is undoubtedly the big winners for the delayed decision. Fractious party disputes have been headed off for now. Meanwhile, proponents and opponents of airport expansion are left united in one thought:

"Just get on with it."