Britain must embrace nuclear power if it is to meet its commitments on climate change, four of the country’s leading environmentalists – who spent much of their lives opposing atomic energy – warn today.

The one-time opponents of nuclear power, who include the former head of Greenpeace, have told The Independent that they have now changed their minds over atomic energy because of the urgent need to curb emissions of carbon dioxide.

They all take the view that the building of nuclear power stations is now imperative and that to delay the process with time-consuming public inquiries and legal challenges would seriously undermine Britain’s promise to cut its carbon emissions by 80 per cent by 2050.

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The volte-face has come at a time when the Government has lifted its self-imposed moratorium on the construction of the next generation of nuclear power stations and is actively seeking public support in the selection of the strategically important sites where they will be built by 2025.

The intervention is important as it is the first time that senior environmental campaigners have broken cover and publicly backed nuclear power.

It will be a welcome boost to the Government, which is expecting strong protests about the new generation of nuclear power stations at the planning stage.

The four leading environmentalists who are now lobbying in favour of nuclear power are Stephen Tindale, former director of Greenpeace; Lord Chris Smith of Finsbury, the chairman of the Environment Agency; Mark Lynas, author of the Royal Society’s science book of the year, and Chris Goodall, a Green Party activist and prospective parliamentary candidate.

Mr Tindale, who ran Greenpeace for five years until he resigned in 2005, has taken a vehemently anti-nuclear stance through out his career as an environmentalist. “My position was necessarily that nuclear power was wrong, partly for the pollution and nuclear waste reasons but primarily because of the risk of proliferation of nuclear weapons,” Mr Tindale said.

“My change of mind wasn’t sudden, but gradual over the past four years. But the key moment when I thought that we needed to be extremely serious was when it was reported that the permafrost in Siberia was melting massively, giving up methane, which is a very serious problem for the world,” he said.

“It was kind of like a religious conversion. Being anti-nuclear was an essential part of being an environmentalist for a long time but now that I’m talking to a number of environmentalists about this, it’s actually quite widespread this view that nuclear power is not ideal but it’s better than climate change,” he added.

None of the four was in favour of nuclear power a decade ago, but recent scientific evidence of just how severe climate change may become as a result of the burning of oil, gas and coal in conventional power stations has transformed their views.

“The issue that has primarily changed my mind is the absolute imperative of reducing carbon dioxide emissions. Fifteen years ago we knew less about climate change. We knew it was likely to happen, we didn’t quite realise how fast,” said Lord Smith, who described himself as a long-time sceptic regarding nuclear power.

“What’s happened is that we’ve woken up to the very serious nature of the climate-change problem, the essential task of reducing carbon dioxide emissions and the need to decarbonise electricity production over the course of the next 20 to 30 years,” he said.

Renewable sources of energy, such as wind, wave and solar power, are still necessary in the fight against global warming, but achieving low-carbon electricity generation is far more difficult without nuclear power, Lord Smith said.

Mark Lynas said that his change of mind was also a gradual affair borne out of the need to do something concrete to counter the growing emissions of carbon dioxide created by producing electricity from the burning of fossil fuels. “I’ve been equivocating over this for many years; it’s not as if it’s a sudden conversion, but it’s taken a long time to come out of the closet. For an environmentalist, it’s a bit like admitting you are gay to your parents because you’re kind of worried about being rejected,” Mr Lynas said.

“I’ve been standardly anti-nuclear throughout most of my environmental career. I certainly assumed that the standard mantra about it being dirty, dangerous and unnecessary was correct,” he said.

“The thing that initially pushed me was seeing how long and difficult the road to going to 100 per cent renewable economy would be, and realising that if we really are serious about tackling global warming it the next decade or two then we certainly need to consider a new generation of nuclear power stations.”

The long moratorium on building nuclear power plants in Britain came about largely because of intense lobbying by environmentalists in the 1970s and 1980s – a campaign that may have caused more harm than good, Mr Lynas said.

“In retrospect, it will come to be seen as an enormous mistake for which the earth’s climate is now paying the price. To give an example, the environmentalists stopped a nuclear plant in Austria from being switched on, a colossal waste of money, and instead [Austria] built two coal plants,” he said.

The four will now join the ranks of those like Sir David King, the former chief scientific adviser to the Government and now director of the Smith Centre in Oxford, who was sceptical about nuclear power until he was presented with data on the scale of the climate-change problem.