The Sustainable Development Commission, the government's watchdog on sustainable development issues, came down in favour of more renewable energy sources and greater energy efficiency rather than replacing old nuclear plants with new ones.

"We categorically disagree with all those people advising the government that nuclear is necessary," Commission chairman Jonathon Porritt told a news conference.

But he said only eight of the 16 commissioners gave nuclear an unqualified rejection, with five saying no nuclear now and two saying it should remain an option. Porritt, a former head of the Friends of the Earth environmental group, did not vote.

The government is reviewing electricity sources as it closes the ageing nuclear plants which supply 20 percent of the nation's power.

The review is due to report by the middle of the year.

"There is little point in denying that nuclear power has its benefits but, in our view, these are outweighed by serious disadvantages," Porritt said.

He said the nuclear industry had not solved the problem of waste that remains highly toxic for millennia and was very secretive about its costs. Nuclear would risk proliferation and drain money from alternative power and distribution systems.

On the plus side, it was a low carbon technology, had a good safety record in Britain at least and could help secure future energy supplies in an increasingly uncertain world.

But he said even doubling the country's nuclear power generating capacity would only cut carbon emissions by eight percent by 2035, while boosting efficiency could save the equivalent of the carbon output from 27 conventional plants.

Porritt accepted that renewable sources like wind, waves, solar and biomass all had inherent problems - not least that so little investment had actually gone into them so far - and that increasing energy efficiency also denoted changed lifestyles.