Prime Minister Tony Blair has launched a review of UK energy needs which could pave the way for a new generation of nuclear power stations.
Mr Blair says there is "feverish rethinking" over energy policy
He told the Confederation of British Industry renewable sources could fill some but not all energy gaps.
The prime minister is believed to be convinced of the case for nuclear. Business groups want a quick decision but green campaigners oppose the move.
His speech was delayed by Greenpeace protesters in the conference hall.
Mr Blair had to make his speech in a different hall after the protesters climbed into the roof at the Business Design Centre in Islington, London.
Mr Blair said nuclear power was a difficult issue but should be settled by open debate, not protests to stop free speech.
The energy review would be headed by the Energy Minister Malcolm Wicks and report by the middle of next year, he announced.
It would measure the UK's progress against a review carried out two years ago.
And it would "include specifically the issue of whether we facilitate the development of a new generation of nuclear power stations", he said.
Mr Blair said energy policy was "back on the agenda with a vengeance".
"Round the world you can hear the heavy sound of feverish rethinking," he said.
"Energy prices have risen. Energy supply is under threat. Climate change is producing a sense of urgency."
Mr Blair warned that "by around 2020 the UK is likely to have seen decommissioning of coal and nuclear plants that together generate over 30% of today's electricity supply".
"Some of this will be replaced by renewables, but not all of it can," he argued.
The last energy review was in 2003 but since then ministers say Britain has moved from being self-sufficient in gas to being a gas importer.
Mr Blair said the gas market would be "tight" if this winter was as cold as forecasters predicted.
Domestic gas customers and most businesses would not be affected but work was under way to help big gas users face possible problems, he added.
BBC political editor Nick Robinson says the prime minister has been convinced that building more nuclear power stations is the only way to meet energy needs and stick to the targets on climate change.
But Mr Wicks said neither he nor the prime minister had made up their minds.
The minister said his review would look at nuclear power along with renewables, coal, gas and new technologies.
It will also cover energy efficiency and cutting carbon emissions from transport.
Unlike the 2003 review, Mr Wicks said there would be firm proposals on the balance of energy - in other words, a firm yes or no about nuclear.
Last week Mr Blair's chief scientific adviser Sir David King publicly called for the government to give the green light for new nuclear power stations.
Any moves to do so would provoke strong opposition from some Labour MPs.
Ex-Environment Minister Michael Meacher has argued nuclear power is expensive, a terrorist target and produced hazardous waste.
Conservative shadow energy minister Bernard Jenkin said the review was long overdue.
If nuclear was efficient, it should be provided by the private sector. If it was not, the new plants would not be built, said Mr Jenkin.
Liberal Democrat leader Charles Kennedy warned that it had cost £56bn to clean up existing UK nuclear plants.
"The case for revisiting civil nuclear power has not been made by what the prime minister has said and I am very sceptical about this review," he said.
Environmentalists Friends of the Earth are pushing for a programme to reduce electricity waste, be more efficient with fossil fuels and increase renewable energy use.