Much depends on the demand which needs to be met. It is certain that London currently caters for 140 million passengers per year. The trend is certainly upwards, if capacity is available. But could it almost triple to 400 million by 2050 as London's mayor, Boris Johnson, thinks?

At best, any forecast is a guesstimate. The actual number will be determined by many things: the economy, technology, flight patterns, environmental concerns and competition from trains and airports outside of London, and outside the UK. But a prediction is crucial, so the analysis needs to be rigorous and intelligent, independent of any purely political preferences.

The expected demand then needs to be set against the three possible policy options. First, the government could decide to maximise the existing airport capacity at Heathrow, Gatwick, Stansted and Birmingham. This option might encompass a third runway at Heathrow. It certainly could lead to an integrated transport policy based on high-speed rail links. That could make London a stronger hub while also meeting environmental imperatives.

The second policy option is European. Hub airports in Paris, Amsterdam and Frankfurt already have 14 runways among them - perhaps enough to take care of all the increase in air traffic demand in northwest Europe. Co-operation - coupled with investment in stronger terrestrial transport links to and from London - might spur more economic growth across the region than would come with selfish attention on the UK's hub position.

Finally, if the most robust passenger forecasts are accepted, an all-new Thames estuary airport might be the answer. Favoured by the London mayor, it would provide a pleasingly direct, if hugely expensive, solution. The indirect stimuli could prove very powerful too. And the commitment to a brand new London airport would show that Britain can pull hard on the lever marked "thrust".

CONTEXT NEWS

- Howard Davies, the former director general of the Confederation of British Industry, the UK employers' lobby group, is to lead a commission of enquiry into airport capacity in southeast England, according to reports in London's Evening Standard newspaper. The commission will report in the summer of 2015, a date which is likely to be just after the UK's next general election.

- Justine Greening, the UK's minister in charge of transport policy, was replaced by Patrick McLoughlin in a government jobs reshuffle effected on September 4.

- Greening is opposed to the building of a third runway at Heathrow, London's main airport. The decision to give her to a different department has been interpreted as suggesting the UK's Conservative government is considering a reverse to its current policy. In the run up to the 2010 general election it said it opposed the building of a third runway at Heathrow.

- UK Prime Minister David Cameron said on September 5: "While I do believe we need to establish a form of review that will bring parties together and make a decision about airport capacity, I will not be breaking my manifesto pledge".

(Editing by Edward Hadas and David Evans)

• A Permanent Economic Emergency