This may be the moment, senior ministers say, to capitalise on one of Britain's greatest assets, the 45ft tide that races through the Severn estuary, making it the second best place in the world - after Canada's Bay of Fundy - to harness tidal energy.
By building a barrage, they hope to be able to meet a large chunk of Britain's electricity needs from a single renewable, reliable source. It is just one of a number of clean energy technologies they want to employ to keep the lights on, while cutting back the pollution that causes global warming.
On Wednesday, the Government will publish the next stage of its long-awaited energy White Paper. Until now, debate has focused on whether this will lead to the construction of more nuclear reactors, following Tony Blair's repeated insistence that these hold the key to combating climate change. But the nuclear debate has masked the beginnings of what some experts are calling a renewable energy revolution - the potential scale of which has shocked even Whitehall mandarins.
The revolution is being driven largely by legally binding targets for renewable energy agreed by European leaders in March, to help bring global warming under control. At the time, Tony Blair - who had helped to get them agreed - boasted that they were "groundbreaking, bold and ambitious".
He was not wrong. The targets are much more ambitious than the UK renewable electricity plans set out in Wednesday's White Paper and will mean that they have to be scaled up. Few people - least of all the politicians who agreed the European targets - have fully thought through their implications. And the reality is only now beginning to dawn.
Under the EU agreement they have promised, like other European governments, to meet 20 per cent of national energy consumption from renewable sources by 2020. That seems stretching enough. But because of the difficulty of running cars and heating homes on renewables, most of this target is going to have to be met from the energy used to generate electricity. Experts say that will mean boosting the renewable share of the nation's power from the present 4 per cent to 35 per cent in less than 13 years.
At the same time it is becoming clear that building enough nuclear power stations to replace those that will be taken out of service through old age in the next decade is going to be much harder than the outgoing Prime Minister fondly believed. At present they generate 19 per cent of our electricity: by 2020 this could be as low as 5 per cent unless new ones are built.
But constructing atomic power stations is expensive and slow, and has a history of mammoth time and cost overruns. Investors are not rushing to take up the opportunity, and Gordon Brown has made it clear that there will be no subsidies.
With the changeover at No 10 both the plans and the rhetoric are changing. Ministers are determined to keep the nuclear option open, knowing it may be needed, but are placing their hopes in energy saving and renewables, both of which feature heavily in the White Paper.
They are concentrating on three main options - the Severn barrage, another form of tidal energy being already tried out off the Devon coast, and windpower. Energy experts reckon that by 2020 about a fifth of the country's electricity will need to come from wind farms with about another tenth coming from hydroelectric power and tidal power. Wavepower - though potentially a huge resource - is lagging behind.
The advantage of the tides is they rise and fall like clockwork. The Severn barrage - proposed by a consortium of six leading companies, banded together in the Severn Tidal Power Group - would let the water in as it rose, and release it through sluices to power some 216 turbines as it fell - generating about 5 per cent of Britain's electricity.
A similar barrage - some 36 times smaller - has been generating electricity without a hitch at La Rance, Brittany, for three and a half decades. The problems, rather, are environmental. The huge structure would dramatically change the ecology of the estuary: nearly a fifth of Europe's ringed plover and 7 per cent of its Bewick swans rely on the Severn.
Environmentalists back an alternative proposed by Tidal Electric - a series of lagoons, the first of which would be built in Swansea Bay. These would not alter the estuary and might even provide new wildlife havens. Preliminary studies suggest they could provide more energy, and be built faster - but more work needs to be done.
The Government's second tidal power project is far less controversial. A pilot tidal stream power scheme - using a giant submarine windmill whose blades are slowly turned to generate power by the ebbing and flowing tides - is already in operation off Lynmouth in Devon. Senior Government sources believe that arrays of them - installed off Scotland and Northern Ireland, in the Channel Islands and off Portland Bill - could provide at least as much electricity as the barrage.
By contrast windpower can be relied upon to raise a storm - and a celebrity one at that. Thirty years on, the former presenters of rival Saturday morning shows BBC's Swap Shop and ITV's Tiswas are at loggerheads again.
Noel Edmonds is chairman of the Renewable Energy Foundation (REF), which opposes onshore wind farms. "We are spending money like water and making [renewable energy] speculators rich for very little planetary gain," he says. But Chris Tarrant calls wind turbines "modern-day guardian angels" and wants more of them.
At present less than 2 per cent of our electricity comes from windpower, making Britain one of Europe's laggards even though it has its best wind resources. Another 10,000 new wind turbines may have to be built to generate enough electricity, which the Campaign for the Protection of Rural England says could take up an area almost the size of Exmoor.
Objections, and Britain's planning system, are holding up development. In the four months following the publication of the Stern report on climate last October last year, only a third of applications for onshore wind farms were approved, as compared to three-quarters of all major planning applications.
The Government tomorrow publishes a White Paper designed to speed up planning, but it is also placing its hopes in giant windfarms offshore. These are less controversial but more expensive, and Dave Farrier, head of renewables at E.on energy group says that without improvements in subsidies "the rate of deployment of offshore developments will remain relatively slow".
And yet there seems no alternative. Unless ministers seize the time, and tide, they are likely - as Brutus warned - to end up "bound in shallows and in miseries".
Meeting Britain's renewable energy commitments
Britain has a long way to go to meet its renewable targets by 2020. Experts say that at least a fifth of the UK's electricity would have to come from wind power (about 30gw of capacity) and another tenth from marine and hydro energy.
Less than 2 per cent currently comes from wind power. About 25gw of wind farms are in various stages of planning but many of these will not get built.
The biggest wind farms are offshore. More wind farms than those shown on the map would need to be built to meet Britain's targets by 2020.
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