A demolition job: Boris’s credibility crashes in the Thames estuary

This was a demolition job. Today's Airports Commission report on Boris Johnson's estuary airport proposals does not so much dismiss his plans as put 3,000 tonnes of explosives under them.

Commission chair Howard Davies describes the economic risks of Johnson's proposals as "huge," and the delivery risks as "very great".  The environmental hurdles to building an airport in the Thames estuary are described as potentially "impossible" to overcome and the costs as astronomical.

At a price tag of up to £120 billion, it would be the equivalent of building another eight Crossrails. An unimaginable price for any government.

As Davies notes: "We cannot see that additional infrastructure investment in the South East, on the scale implied, with uncertain economic benefits, would be likely to appeal to the chancellor of the exchequer in a government of any political colour."

This is to put it mildly. There is no government, save one run by Johnson himself, that would even consider this project again following today's report.

Davies' demolition job continues. He finds the scheme would extend journey times to the UK's hub airport by up to 26% and lead to the closure of not just Heathrow but possibly Southend and City Airport as well.

These closures would put hundreds of thousands of jobs at risk, with Heathrow alone supporting around 200,000 jobs in the surrounding area.

For these reasons and many more the commission found that Johnson's proposals were hugely unpopular.

"The aviation industry doubts the viability of the plan, local councils are opposed, and business groups are similarly unenthusiastic," he notes, adding that "few people outside the direct advocacy groups support the idea."

For "advocacy groups" read Boris Johnson and City Hall.

This was always an idea that had little support outside of Johnson's own head. It was a colossal vanity project that in any sensible administration would have been killed off years ago.

Instead Johnson has ploughed ahead for six long years with an idea that had no obvious support among airlines, the government, or the public.

Just months ago when it was already clear that the Commission were going to junk his proposals, Johnson signed off an extra £2 million worth of expenditure on lobbying for his plans, bringing the total bill to London taxpayers to over £5 million.

Johnson's devotion to the project has long remained a mystery. The idea of building an airport in the estuary is not new and has been raised and rejected repeatedly in one form or another for over forty years.

The idea was resurrected for a final time by Johnson's deputy Kit Malthouse. Malthouse, a businessman and one-time wannabe aviation entrepreneur turned the mayor on to the idea back in 2008. Back then, the proposal was to build an airport on a man-made island in the outer Thames estuary.

This idea would have been even more expensive than the far more modest proposals rejected by Davies today.

In any case, almost from day one it was clear that the scheme was a non-starter. Conservative councils and MPs in Kent immediately mobilised against the proposals as did an unlikely coalition of environmental groups and aviation lobbyists. Labour and the Lib Dems rejected it and few senior figures in the Conservative party showed even a token amount of enthusiasm for the idea.

Any other politician would have quietly ditched the scheme and moved on, but even now Johnson is refusing to believe his dream is over. Faced with today's devastating report, the mayor has exploded with rage, with those around him claiming that the only thing sunk by the report is the credibility of the commission. In a remarkable statement, Johnson insisted he was "absolutely certain" the scheme would still go ahead.

There is a tinge of megalomania about the mayor's continuing obsession with his airport. A kind of raging impotence at his inability to tarmac over huge areas of natural beauty on a mayoral whim.

Whatever the cause of this, the saga has done little for his attempts to appear as a credible future prime minister.

For now though, the main reaction among campaigners against the airport will be relief. For six years Johnson's fevered vision has hovered over people living in the estuary like an alien invader. Now finally they can get back on with their lives.