Mark Leftly: Boris Johnson's airport plan is a monumental folly without a grain of sense

Westminster Outlook Boris Johnson's plan to replace Heathrow Airport with a four-runway mega-hub on the Isle of Grain in Kent is a curious thing. The London mayor's own submission to the Airports Commission last summer concedes: "National Grid's LNG (liquefied natural gas) facility… on the south-east side of the island, is too tall and too close to the proposed airport site."

In a pre-9/11 world, building a 55 sq km airport right next to Europe's biggest LNG importation terminal and the largest above-ground storage tanks of their kind in the world would have seemed a bit daft.

Today, it is an irredeemably irresponsible suggestion, particularly given Mr Johnson's farcical excuse for a solution: it's ok, the LNG plant could just be moved somewhere else (presumably National Grid would be told that the removal vans were coming).

In their 35-page submission, a half-sentence is all Mr Johnson's team could conjure to explain how they would deal with this figurative and literal obstacle to the mayor's flight of fancy.

No suggestions, no detail on where a facility that could supply one-fifth of the UK's future gas requirements might reasonably go or whether this was even feasible. Those LNG operations are not a funfair; they cannot just be dismantled and moved to the next available site at the first blustering behest of an admittedly formidable performer-politician.

This is what sensible business executives, MPs and aviation experts have been thinking, even before they started probing assumptions that the mayor's advisers have made about the costs, flight paths and environmental impacts of the airport.

Mr Johnson is, of course, a polarising figure, who succeeds politically by defining himself against his opponents and who is holding true to his conviction that there should not be expansion at Heathrow. He has cast himself as something of a visionary, which, as invented convention rather than common sense dictates, means that others are left to fill in the minutiae of his grand plans.

What's really interesting is whether Sir Howard Davies, a former Financial Services Authority chairman, and his Airports Commission privately shared the views of Mr Johnson's many opponents.

Last summer, there was a very strong rumour that the commission had ruled out the Isle of Grain as a suitable site for a future airport largely because of the proximity to the LNG facility. Ahead of the publication of last month's interim report, it was widely believed that a Thames Estuary option would be completely ruled out.

The initial short list did indeed focus on options for expanding Heathrow, as well as Gatwick. But Sir Howard left Mr Johnson with a glimmer of hope, as the commission stated it would take another look at whether or not the Isle of Grain was a "credible proposal", which was launched yesterday.

This led to one more bit of speculation. An unpredictable, crusading Mr Johnson could prove politically dangerous, while the Conservative Party ran on a 2010 general election manifesto that stated "our goal is to make Heathrow airport better, not bigger".

Could it be that ministers and officials pressured the commission to at least allow the faint possibility of a Thames Estuary airport, given the likely outcry against its support for Heathrow?

The good news is that Sir Howard will be afforded the opportunity to answer that very question when he faces the powerful transport select committee on Monday.

Should Sir Howard definitively deny these rumours, then committee chair Louise Ellman and her colleagues must demand to know why on earth a proposal of such monumental folly was not precluded at the first opportunity.