Leading article: A new airport on the Thames may be the answer for London

Those who have been following the evolution of Conservative Party policy under David Cameron might have sensed the way the wind was blowing, but yesterday's announcement still came as a surprise. Addressing the party conference in Birmingham , the shadow Transport Secretary, Theresa Villiers, pledged that a Conservative government would tear up plans for a third runway at Heathrow airport and use the money to build a high-speed, north-south rail line instead.

The surprise derives in part from the image of the Conservatives as the business-friendly party and the fact that business interests have lobbied strongly for the third runway. It stems also from the impression given by the present, Labour, government that approval for the third runway is already a done deal. There was thus no need for the Conservatives to make this a party political issue; they could have treated the third runway, gratefully, as a fait accompli.

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Instead, the Conservatives have tapped into a public mood that questions the need for more airport space and projections that growth in air travel has stalled. According to their figures, a new high-speed rail line linking Leeds with Heathrow, London and the Channel Tunnel, could reduce the number of Heathrow flights by 66,000 a year.

It is well known that a lot of Heathrow traffic is made up of passengers changing planes. Less well known, perhaps, is the large number of flights from this high-cost, high-density airport that are short-haul and domestic. As the success of France's high-speed railways and the Eurostar have shown, business and leisure travellers alike will choose the train if time and cost considerations are comparable.

But the discussion should not be allowed to stop there. London's mayor, Boris Johnson, has shown the way by reviving the idea of a brand new airport for London in the Thames estuary. That his musings were at once dismissed as unrealisable or undesirable could have been predicted. Formidable vested interests are tied up in the status quo. But it is precisely such fearless, big-picture thinking that is required. After all, whose interests are served by having so much prime development land close to London occupied by an ever-expanding airport? Why should aircraft noise blight so many densely populated urban areas, and how safe is it for so many planes to traverse greater London every hour?

Current circumstances create unusually favourable conditions for new thinking about airport provision in general, and airports in the congested South-east in particular. High fuel prices and the credit crunch have already bankrupted some airlines, caused others to reduce the number of flights and started to curb passenger demand. EU targets for curbing carbon emissions are pressing, as well as the growing public intolerance of air and noise pollution. Meanwhile the requirement for BAA to divest itself of at least one of its airports in the South-east will bring in at least one new player.

Heathrow has for too long been deferred to as an elemental force that brooks no resistance. The result has been a skewing of the region's development, and an airport – Terminal 5 notwithstanding – that needs vast new investment if it is to compete with the world's best. With high-speed trains and many passengers whose chief requirement is to change planes, there is no reason why London's flagship airport has to be at Heathrow and many compelling reasons why it should not. The mayor, we believe, has voiced an idea whose time has come.