Tony Blair has told MPs he has changed his mind in the last three years on the need for new nuclear power stations.
An Energy White Paper in 2003 was sceptical about building new nuclear plants, but left the option open.
A government energy review, due this month, is expected to call for additional nuclear power stations.
Mr Blair said he believed new nuclear stations were needed to tackle climate change but wanted the review to decide whether he was right or not.
No other cure?
Appearing before the Commons liaison committee, the prime minister denied suggestions that he had prejudged the energy review.
"If the review had come out with evidence that this was a bad idea, then of course my mind would have been differently made up," he said.
"Unless someone can show through energy efficiency and renewables you're going to be able to cure the whole of this problem then I think that nuclear power goes back on the agenda"
Mr Blair said he had come to view that it was difficult to see how Britain could have secure energy supplies without new nuclear power stations to replace the older plants when they are decommissioned.
That was why he had asked for the energy review.
He promised that the review would be "every bit as radical" in its proposals for improving energy efficiency and increasing renewable sources of energy.
Mr Blair said his changing attitude to nuclear power since the 2003 White Paper had been the result of energy prices doubling or trebling amid worries about security of supplies, and the issue of climate change.
He said the whole issue had risen up the agenda globally, with it discussed at every European Council meeting, as well as the forthcoming G8 gathering.
'Big solutions'
"Unless someone can show through energy efficiency and renewables you're going to be able to cure the whole of this problem then I think that nuclear power goes back on the agenda," he said.
But Liberal Democrat environment spokesman Chris Huhne said Mr Blair's "prejudgement" of the energy review underlined "his infatuation with big solutions rather than pragmatic ones".
"Nuclear is a tried, tested and failed technology," he said.
"Not a single nuclear plant has been built anywhere in the world by private investors without lashings of government subsidy since Chernobyl and Three Mile Island.
"Other countries like Germany and Sweden have opted for a non-nuclear future and are making good progress with energy saving and renewables."
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