Climate change demands Britain consider building new nuclear power plants, says Lord May, the Royal Society president.
Radioactive waste is the issue which dogs nuclear
The government's former chief scientist told the Daily Telegraph that the UK would struggle to reduce its emissions of carbon dioxide without nuclear.
He said politicians must be courageous and start talking about the "unpopular child in the energy family".
He argued that the idea Britain could meet its energy needs with renewables alone was simply wishful thinking.
The head of the UK's national academy of science was reacting to the keynote environment speeches given this week by Prime Minister Tony Blair and Conservative opposition leader Michael Howard.
Both recognised the potential disaster facing the planet if CO2 emissions grew unchecked but neither mentioned building new nuclear power stations as a possible solution to the problem.
'So much at stake'
Challenged directly on the subject by BBC News Online after his address, Mr Howard said: "Only a government can take that decision... We'll decide when we return to government."
Mr Blair reiterated his position in the Commons on Wednesday when he said the government had an open mind.
"We have made it clear in the White Paper that we published that we haven't shut the door on [nuclear] but until the issues to do with cost and public concern over safety can be met there is simply not the consent to go ahead with it," he said during Question Time.
Lord May: Time to act is now
But Lord May said the politicians' lack of courage on the issue was disappointing.
"The truth is that it will be difficult for Britain to lead the way on climate change in the mid-term future without building new nuclear power stations," he wrote in his Telegraph opinion column.
"Looking to the future, we need to be aiming for reductions in carbon dioxide emissions of about 60% by the middle of this century to avoid the worst-case scenarios for climate change.
"Yet Britain's emissions actually rose by 1.5% between 2002 and 2003.
"Many of us want to believe in the promise of largely benign renewable energies, such as wind and solar, to satisfy completely our seemingly insatiable appetite for energy at low cost to the environment. But now, when there is so much at stake in averting a climate crisis, is not the time to retreat into wishful thinking."
Lord May is among several leading thinkers to raise the issue of a new nuclear building programme.
Professor James Lovelock - who developed the Gaia Hypothesis of a benign, self-regulating Earth - outraged the environmental lobby earlier this year when he also called for nuclear to play a part in tackling climate change.
And Sir Crispin Tickell, formerly the UK's ambassador to the United Nations, has accused British politicians of failing to give a lead on nuclear energy.
But any move to extend the lives of nuclear power stations, let alone build new ones, is likely to provoke strong public opposition.
Sizewell B in Suffolk, a pressurised water reactor, was the last nuclear plant to come into operation in the UK, in 1994. It took seven years to build, after the largest ever public enquiry in Britain.
Many of the country's other stations are now approaching the end of their lives.
Roger Higman, a climate change campaigner for Friends Of The Earth, rejected Lord May's analysis.
He described nuclear power as an "expensive, dirty option".
"It is intimately associated with the technologies that are used to make nuclear weapons," he told BBC News Online.
"If we are going to be using nuclear to combat climate change, it will be impossible to persuade anybody else to reduce their emissions without giving them access to nuclear power... and that also enables them to build bombs."
Mr Higman said the UK had superb renewable potential in wind, wave and tidal power.
"I think it's rather sad that for years Britain has been advised by people who don't see what the rest of Europe is doing on renewable energy, and isn't following it up."