One of the world's most prominent scientific figures to be sceptical about climate change has admitted to being paid more than $1m in the past decade by major US oil and coal companies.

Dr Willie Soon, an astrophysicist at the Solar, Stellar and Planetary Sciences Division of the Harvard-Smithsonian Centre for Astrophysics, is known for his view that global warming and the melting of the arctic sea ice is caused by solar variation rather than human-caused CO2 emissions, and that polar bears are not primarily threatened by climate change.

But according to a Greenpeace US investigation, he has been heavily funded by coal and oil industry interests since 2001, receiving money from ExxonMobil, the American Petroleum Insitute and Koch Industries along with Southern, one of the world's largest coal-burning utility companies. Since 2002, it is alleged, every new grant he has received has been from either oil or coal interests.

In addition, freedom of information documents suggest that Soon corresponded in 2003 with other prominent climate sceptics to try to weaken a major assessment of global warming being conducted by the UN's leading climate science body, the Nobel prize-winning Intergovernmental Panel on Climate Change.

Soon, who had previously disclosed corporate funding he received in the 1990s, was today reportely unapologetic, telling Reuters that he agreed that he had received money from all of the groups and companies named in the report but denied that any group would have influenced his studies.

"I have never been motivated by financial reward in any of my scientific research," he said. "I would have accepted money from Greenpeace if they had offered it to do my research." He did not respond to a request from the Guardian to comment.

Documents provided to Greenpeace by the Smithsonian under the US Freedom of Information Act (FoIA) show that the Charles G Koch Foundation, a leading provider of funds for climate sceptic groups, gave Soon two grants totalling $175,000 (then roughly £102,000) in 2005/6 and again in 2010. In addition the American Petroleum insitute (API), which represents the US petroleum and natural gas industries, gave him multiple grants between 2001 and 2007 totalling $274,000, oil company Exxon Mobil provided $335,000 between 2005 and 2010, and Soon received other grants from coal and oil industry sources including the Mobil Foundation, the Texaco Foundation and the Electric Power Research Institute.

As one of very few scientists to publish in peer-reviewed literature denying climate change, Soon is widely regarded as one of the leading sceptical voices. His scientific position and the vehemence of his views has made him a central figure in a heated political debate that has informed the US right wing and helped to undermine public trust in the science of global warming and UN negotiations.

"A campaign of climate change denial has been waged for over 20 years by big oil and big coal," said Kert Davies, a research director at Greenpeace US. "Scientists like Dr Soon, who take fossil fuel money and pretend to be independent scientists, are pawns."

Soon has strongly argued that the 20th century was not a uniquely extreme climatic period. His most famous work challenged the "hockey stick" graph of temperature records published by Michael Mann, which showed a relatively sharp rise in temperatures during the second half of the 20th century. A paper published with Sallie Baliunas in 2003 in the journal Climate Research which attacked the hockey stick on flimsy evidence led to a group of leading climate scientists including Mann deciding to boycott the journal. In a letter to the Guardian in February 2004, Soon wrote that the authors had been open about their sources of funding. "All sources of funding for our research were fully disclosed in our manuscript. Most of our funding came from federal agencies, including the Air Force Office of Scientific Research and Nasa," he wrote.

He has also questioned the health risks of mercury emissions from coal and in 2007 co-wrote a paper that down-played the idea that polar bears are threatened by human-caused climate change

The investigation is likely to embarrass Exxon, the world's largest oil company, which for many years funded climate sceptics but in 2008 declared it would cut funds to lobby groups that "divert attention" from the need to find new sources of clean energy. According to the documents, Exxon provided $55,000 for Soon to study Arctic climate change in 2007 and 2008, and another $76,106 for research into solar variability between 2008 and 2010.

Exxon spokesman Alan Jeffers said this week the company did not fund Soon last year, and that it funds hundreds of organisations to do research on climate and the environment.

Southern gave Soon $120,000 starting in 2008 to study the Sun's relation to climate change, according to the FIA documents. Spokeswoman Stephanie Kirijan said the company has spent about $500m on funding environmental research and development ,and that it did not fund Soon last year.

In one 2003 email released to Greenpeace, that Soon sent, it is believed, to four other leading sceptics, he writes: "Clearly [the fourth assessment report] chapters may be too much for any one of us to tackle them all ... But as a team, we may give it our best shot to try to anticipate and counter some of the chapters ..." He adds: "I hope we can ... see what we can do to weaken the fourth assessment report."

In 2003 Soon said at a US senate hearing that he had "not knowingly been hired by, nor employed by, nor received grants from any organisation that had taken advocacy positions with respect to the Kyoto protocol or the UN Framework Convention on Climate Change."