Climate Change Secretary Ed Miliband has clashed with former chancellor Lord Lawson over global warming, ahead of the Copenhagen summit.
They appeared on the BBC's Politics Show, where Lord Lawson, chairman of the Global Warming Policy Foundation, repeated his long-held doubts.
He said there had been "no further warming" since the late 20th Century and dissent should be tolerated.
Mr Miliband accused Lord Lawson of being "profoundly irresponsible".
The energy secretary said Lord Lawson was "spreading doubt" despite a scientific consensus.
"What we've heard from the national academies of the eight leading industrial countries, and the five leading emerging countries, like China and India, is that global warming is man-made and it's happening," he said.
"I think you are being profoundly irresponsible by saying we can stick our heads in the sand and just hope this thing goes away. It's not going to."
Lord Lawson said it was also profoundly irresponsible not to allow an "honest, rational, reasoned debate".
He said there were eminent scientists on both sides of the debate and he believed global warming "appeared not to be happening at the present time".
"There was a little bit of warming in the last quarter of the 20th Century, half a degree centigrade; there's been no further warming since then," he said.
However, he acknowledged "there may well be further warming".
Lord Lawson said his think tank, the Global Warming Policy Foundation, was concerned with the "extremely damaging and harmful policies" put forward to deal with global warming.
He said they would cost an "enormous amount" and would be "damaging" for poor countries which needed carbon-based energy for economic development.
Mr Miliband admitted so-called "ClimateGate" had been "damaging".
He said the leaked e-mail exchanges between experts, which prompted sceptics to claim data had been manipulated, had been used to try to "sabotage" the Copenhagen summit.
"My answer to it is maximum transparency, let's get the data out there," he said. "The people who believe that this is happening, that climate change is happening and man-made, have nothing to fear from transparency."
The Copenhagen summit, which is aimed at reaching a new global deal on climate change, is due to get under way on Monday.
On Saturday, tens of thousands of people took part in demonstrations around the world, calling on world leaders to reach a tough new deal on cutting carbon emissions.
Gordon Brown, who met some of the demonstrators in Downing Street, said he and the "vast majority of people" were convinced by the scientific evidence for man-made global warming.
"There's a flat earth group over the evidence, if I may say so, that exists about climate change, and we've got to show them that the scientific evidence is strong," the prime minister said.
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