The evidence that climate change will cause immense damage grows stronger, the need to combat it more urgent. But Britain's ageing reactors are being shut down. By 2020 only one will be left, and the atom's share of our electricity will have shrunk from about 20 to 4 per cent. To have any hope of tackling global warming we will have to replace them.

The logic seems unassailable and - fanned by a nuclear industry PR campaign - it has swept through much of the Cabinet and the commentariat. But as it has grown fashionable, I have become sceptical. The more I have looked into it, the more I have come to believe that building new reactors will not only fail to reduce global warming, but actually make it worse. For me this outweighs the traditional objections to atomic power, though as a longtime critic of the industry's performance - albeit one believing that the nuclear option should be kept open - I accept they have force.

Solving the problem of what to do with nuclear waste - which will remain dangerous for 240,000 years, 20 times longer than the entire history of civilisation - is a moral imperative. It cannot be left for our descendants to sort out.

A serious accident could be far worse than the Chernobyl catastrophe, which happened in benign weather that prevented most of the radioactivity from falling out nearby. New reactors are designed to be much safer, but cannot provide complete protection against human error, the cause of almost every accident, or near miss, so far. And a terrorist attack on a nuclear power station, waste store or shipment grows ever more likely: only this month a planned attack on an Australian reactor was uncovered.

Nevertheless, the dangers of global warming are now so enormous that I would accept these hazards if, as proponents claim, expanding nuclear power would avert, or even significantly reduce, climate change.

In fact, it was none of these objections - but simple economics - that caused nuclear construction to decline after peaking 30 years ago. As even No 10 admits, not a single reactor has been built in a liberalised market anywhere in the world.

Cost still remains the greatest obstacle. Mr Blair's own Performance and Innovation Unit concluded that nuclear power was about twice as expensive as generating electricity from gas or wind. Investors still spurn it. Even the nuclear industry does not want to build reactors without government assistance.

Chancellor Gordon Brown is deeply reluctant to provide this, even though he is increasingly concerned about global warming, recently seeing off an attempt from the Prime Minister's aides to abandon the Government's commitment to cut emissions of carbon dioxide by 20 per cent by 2010.

Yet, again, if more nuclear was the best way to combat global warming - or even an important contribution - narrow economics should not be allowed to get in the way. But this is far from so. For a start, atom ic power is not, as is claimed, "carbon free". It may not emit carbon dioxide from its power stations, but huge amounts are produced from the energy burned in processing the uranium that fuels them. In all, it is estimated, it now generates a third as much as gas-fired electricity. And this will rise as poorer uranium ores are used.

Next, nuclear cannot do much. It is used only in electricity generation, which presently emits only about a quarter of Britain's carbon dioxide. And it will produce only a relatively small amount of the country's power. So even building 10 new reactors, as the industry wants, will tackle only about 8 per cent of the problem. And it cannot do even that in time. The industry itself admits that it will take 10 years to get the first new reactor on stream. More sceptical experts put the time at around the 15 years it took to build Britain's last one, at Sizewell.

Yet, the crucial period for reducing emissions, if there is to be any chance of controlling global warming, is the next decade. A decision to resume building nuclear power stations risks stifling the two things that could make a difference: windpower and energy saving.

While controversy has raged about it, the wind industry has quietly achieved a quantum leap. So far this year it has erected 500 MW of turbines and has planning approvals to enable it to keep up the pace. At that rate it will build the equivalent of a new big atomic power station every two years.

Energy saving can offer much more, and provides the only real hope for tackling global warming quickly enough. Britain could cut its energy consumption by a third without any loss of comfort or economic growth. And every pound spent on conservation saves seven times as much carbon dioxide as one devoted to nuclear power.

Yet witness after distinguished witness to an inquiry by the House of Commons Environmental Audit Committee has testified that a decision for new nuclear build will inevitably deprive windpower and energy saving of the capital and political commitment they need. This has already begun to happen in Finland, the first European country to restart building reactors.

Sir Jonathon Porritt, Mr Blair's environmental adviser, says that once a pro-nuclear decision was made government and industry "will not think any more about renewables and energy efficiency".

We would, in effect, be putting all our eggs in one basket - and, appropriately, they would be fragile. A single nuclear accident, anywhere in the world, would smash them.

Professor Sir David King - the Government's chief scientist, and consistently its global warming conscience - supports new reactors because he says we will need "every tool in the bag" to fight the climate change. But if he and Tony Blair get their way, we will be left with only one tool, and a blunt one at that.