Boris Johnson's proposal of a new airport in the Thames Estuary, now immortalised as "Boris Island", was once ridiculed. But the news, revealed in this morning's Daily Telegraph, that David Cameron will soon announce a formal consultation on the plan means we can no longer be so dismissive. The Prime Minister, hitherto a sceptic, has become a possible convert following representations by business leaders.

Indeed, the government was originally due to announce its provisional backing for the plan on 3 January but was blocked by a concerned Nick Clegg. The word from the Lib Dems, ever keener to differentiate themselves from the Tories, is that the Deputy PM is "not in favour of it but is prepared to allow a consultation". As well as the usual environmental objections, the party is uneasy about a plan that is handily timed to coincide with the Mayor of London's re-election bid.

So, while the ultimate fate of Boris island remains unclear, the mere announcement of a consultation is something of a coup for the Mayor, who has often been a lonely advocate for the plan. He was in ebullient form on the Today programme this morning, cheerfully announcing that "Dave" and "George" were "increasingly interested" in the plan. Expanding Heathrow, he declared, was not an option:

The trouble with [expanding Heathrow] is 25% of people in Europe who suffer from aviation noise live near Heathrow, in other words we have one of the worst problems. It's fundamentally in the wrong place and if we build a third runway we'll be entrenching that fundamental mistake. We'll be causing massive congestion in west London and that's why I think the government, George and Dave, are absolutely right to start looking at a more imaginative solution.

Unlike "Boris bikes", which were his predecessor's idea, this is one grand projet that the Mayor can truly claim as his own.