Twelve miles off the coasts of Kent and Essex lies a patch of shallow sea for which the Government has grand plans. It hopes that private investors will find at least £3 billion to erect more than 340 giant wind turbines in the Thames Estuary over the next three years. According to its designers the London Array would be the world's biggest off-shore wind farm, producing up to 1,000 megawatts hours of power or enough to light 750,000 homes. It would play a critical role in meeting Britain's self-imposed target of producing 35 per cent of its power from renewable sources by 2020. The trouble is, it may never be built.

The same goes for other offshore wind power projects budgeted at a further £7 billion that, until the credit crunch, were destined to bring ghostly views to the wilder shores of Britain and clean power to its people. These projects have stalled, victims of the scarcity of credit and high cost of imported hardware brought on by the recession, and above all by the low cost of dirty, old-fashioned fossil fuels.

The Government's policy on renewables has run aground. The proportion of British power coming from sustainable sources is rising slowly, but in 2007 only 1 per cent of it was derived from wind. Gas is by far the country's fastest-growing source of energy, while global investment in renewables has followed both the national trend and those of the financial markets: it slumped by more than half in the past year. As if to emphasise the mockery the recession has made of efforts simultaneously to boost the economy and the environment, Lord Mandelson's plan for £2,000 scrappage fees for old, polluting cars has promptly been blocked by the Treasury. The money, it appears, cannot be found.

The middle of an historic downturn is not an auspicious time to attempt a large-scale switch from gas to wind generation when the latter costs up to four times as much per kilowatt as the former. Even so, Barack Obama has earmarked tens of billions from his US stimulus programmes for the construction of a green energy infrastructure. Gordon Brown appears determined to follow suit: he has promised “thousands of green jobs” in the forthcoming Budget, and many would be created if investors can be induced to drag the London Array off the drawing board and into the sea.

One way of achieving this - which the private sector is promoting and the Government is understood to be considering - would be to double the number of tradable renewables obligation certificates (ROCs) awarded to wind power investors per unit of electricity they produce. ROCs are this Government's chosen incentive mechanism to boost investment in renewables. Paid for by high-carbon energy producers, and ultimately by consumers, they have so far failed to deliver the capacity that direct grants have created in Germany and Denmark.

A more generous allotment of ROCs to investors might change that, cutting net carbon emissions and boosting diversity of energy supply in the process. But ROCs are by no means the whole answer. Applied only to wind, they do not guarantee the most efficient use of taxpayers' and consumers' money, which should also be available for tidal and solar projects as those technologies evolve. Nor do they remove planning obstacles that can delay and derail vital projects.

If the Government is serious about meeting its renewables targets, it cannot bet only on wind. It must streamline planning legislation radically and invest heavily in the neglected national grid so that new offshore wind farms can connect swiftly to it. It must also lead efforts to lower the overall carbon cap in the next phase of Europe's cap and trade scheme. Until then, renewables will unfortunately be unaffordable.