Gordon Brown was urged last night to bring back genetically modified crops and usher in a new era of nuclear power as part of a wide-ranging package to tackle the burden of population growth and climate change.

Sir David King, the government's chief science adviser, said GM technology was "crucial" for a green revolution capable of providing enough food for the global population, which is due to exceed 9 billion by 2050. He added that new nuclear power stations should be built to help meet carbon emission targets until alternative green energy sources become available.

King's comments came in a valedictory speech at the Royal Society yesterday evening. He is due to relinquish his post at the end of the year, making way for John Beddington, a biologist at Imperial College London.

The speech follows recent revelations in the Guardian that the government has given its backing to a fresh campaign by farmers and industry to introduce GM crops to Britain. In September, the Guardian reported that ministers believed public concerns over GM had softened.

King's stance on GM technology incensed green campaigners yesterday. They claimed the crops had failed to make farming sustainable and had succeeded only in giving a handful of multinational companies greater control over global food production.

King urged the prime minister to back GM and nuclear energy to tackle the major challenges of the 21st century. "By 2050 we will need to feed over 9 billion people on the planet. We will, I believe, only do this with the assistance of a third green revolution, and GM technologies will be crucial in delivery of this," he said.

"To date, the government has taken a broadly neutral approach to GM issues, with its priorities being to protect human health and the environment and provide choice for the consumer over whether or not to purchase GM food. I believe that it's now time to revisit this issue."

The chief scientist, who was parachuted in by Tony Blair in 2000 and quickly became involved with tackling the foot and mouth crisis, has long advocated the use of GM and nuclear, but this is his first call for the government to act. "It is now time to give the green light to nuclear energy. While I have high hopes for new zero-emissions technologies in the future, efficient nuclear fission power stations are already available. I am also hopeful that fusion power stations, without the problems of nuclear waste disposal, will emerge over the coming three or four decades," he said.

In January 2004, King was widely praised for attacking President George Bush's failure to tackle climate change, which he described as a more serious threat than terrorism.

The comments in King's parting speech were rejected by the green campaigner group Friends of the Earth, which said agricultural biotechnology was not the solution to environmental problems. "Despite 30 years of research ... GM crops have failed to deliver the sustainable solutions that are urgently needed," said Claire Oxborrow, a GM campaigner with the organisation.

"We need to urgently tackle the environmental challenges. This requires investment in worldwide sustainable farming methods which meet local environmental and social needs, a reduction in meat and livestock production and a freeze on the rush to develop agrofuels."

King also took the opportunity to criticise the poor level of scientific understanding among ministerial advisers and warned them not to cherry-pick scientific information before briefing ministers.