Ex-environment minister Michael Meacher has told Tony Blair it was "patently untrue" that nuclear power was needed for the UK to meet emissions targets.
Mr Blair says a debate is needed on nuclear power
The prime minister says nuclear power could improve the security of the UK's energy supply and help climate change.
But Mr Meacher said "steady expansion" of renewable energy could allow the UK to meet its greenhouse gases targets.
Nuclear power was more expensive than coal and oil, produced dangerous waste and was a terrorist target, he said.
Mr Meacher, who was sacked as environment minister in 2003, spoke out as the government prepares to announce a review of energy policy, including nuclear power.
He asked Mr Blair why he had told the Commons Liaison Committee, of 31 committee chairman, on Monday that the government has got to "take on" nuclear power.
He said he was "profoundly disturbed" by suggestions that support for targets and timetables in the successor agreement to Kyoto could be dropped.
Mr Blair conceded that a "proper and thorough debate" on the UK's nuclear power programme was needed.
He added that by 2020 the amount of energy from nuclear power will decline "from just over 20% down to 4% and to try to meet the whole of that through the renewables is going to be difficult".