The number of British people who are sceptical about climate change is rising, a poll for BBC News suggests.
The Populus poll of 1,001 adults found 25% did not think global warming was happening, an increase of 10% since a similar poll was conducted in November.
The percentage of respondents who said climate change was a reality had fallen from 83% in November to 75% this month.
And only 26% of those asked believed climate change was happening and "now established as largely man-made".
The findings are based on interviews carried out on 3-4 February.
In November 2009, a similar poll by Populus - commissioned by the Times newspaper - showed that 41% agreed that climate change was happening and it was largely the result of human activities.
"It is very unusual indeed to see such a dramatic shift in opinion in such a short period," Populus managing director Michael Simmonds told BBC News.
"The British public are sceptical about man's contribution to climate change - and becoming more so," he added.
"More people are now doubters than firm believers."
The Department for Environment, Food and Rural Affairs' (Defra) chief scientific adviser, Professor Bob Watson, called the findings "very disappointing".
"The fact that there has been a very significant drop in the number of people that believe that we humans are changing the Earth's climate is serious," he told BBC News.
"Action is urgently needed," Professor Watson warned.
"We need the public to understand that climate change is serious so they will change their habits and help us move towards a low carbon economy."
Of the 75% of respondents who agreed that climate change was happening, one-in-three people felt that the potential consequences of living in a warming world had been exaggerated, up from one-in-five people in November.
The number of people who felt the risks of climate change had been understated dropped from 38% in November to 25% in the latest poll.
During the intervening period between the two polls, there was a series of high profile climate-related stories, some of which made grim reading for climate scientists and policymakers.
In November, the contents of emails stolen from a leading climate science unit led to accusations that a number of researchers had manipulated data.
And in January, the Intergovernmental Panel on Climate Change (IPCC) admitted that it had made a mistake in asserting that Himalayan glaciers could disappear by 2035.
All of this happened against the backdrop of many parts of the northern hemisphere being gripped by a prolonged period of sub-zero temperatures.
However, 73% of the people who said that they were aware of the "science flaws" stories stated that the media coverage had not changed their views about the risks of climate change.
"People tend to make judgements over time based on a whole range of different sources," Mr Simmonds explained.
He added that it was very unusual for single events to have a dramatic impact on public opinion.
"Normally, people make their minds up over a longer period and are influenced by all the voices they hear, what they read and what people they know are talking about."