A high-profile thinktank founded by the former chancellor Lord Lawson, which has been highly critical of climate scientists and action on global warming, appears to have attracted fewer than 100 members in its first year.

Accounts filed with the Charities Commission and Companies House in the last week show for the first time the extent to which the secretive Global Warming Policy Foundation, founded in November 2009, is funded by anonymous donors, compared with income from membership fees. Its total income for the period up to 31 July 2010 was £503,302, of which only £8,168 came from membership contributions. The foundation charges a minimum annual membership fee of £100 .

The accounts include a statement from Lawson in which he justifies why the foundation, which has called for increased transparency and scrutiny of climate scientists, refuses to reveal the identity of its donors: "The soil we till is highly controversial, and anyone who puts their head above the parapet has to be prepared to endure a degree of public vilification. For that reason we offer all our donors the protection of anonymity."

Lawson added that the foundation does not accept donations from the government, nor "from the energy industry or from anyone with a significant interest in the energy industry". When asked by the Guardian for clarification on what he specifically meant by the term "significant", Lawson declined to elaborate other than to say it is "obviously a matter of judgment and common sense, and goes beyond mere numbers". He added: "The exercise of that judgment is a matter for me in the first instance, but it has to be approved by our eminent board of trustees."

The foundation has been a vocal critic of climate scientists and climate policies since it launched on the eve of the Copenhagen climate summit in December 2009. Lawson's statement in the accounts says such timing was "deliberate", but that it was "entirely fortuitous" that it coincided with the unauthorised release of thousands of emails from climate scientists working at the University of East Anglia's Climatic Research Unit.

The foundation called for a public inquiry into the contents of the emails and Lawson later criticised climate scientists for "fudging" data when he gave testified before the science and technology select committee at a parliamentary inquiry last March. The scientists denied the accusation and a parliamentary inquiry later concluded that there was no evidence to challenge the "scientific consensus" that global warming is induced by human activities. It did, however, scold the university for not tackling a "culture of withholding information".

The accounts also reveal that the foundation's honorary treasurer is Sir James Spooner, a former director of Barclays and J Sainsbury. Asked by the Guardian if he was a donor, Spooner said: "I'm a very small contributor – a four-figure number at the most." He added that the foundation received "no money from companies whatsoever, nor from anybody anywhere near the energy industry in its broadest sense. The foundation is funded by individuals, foundations and trusts, but their identity is confidential".

This month, in a letter seen by the Guardian, Bob Ward, policy and communications director of the Grantham Research Institute on Climate Change and the Environment at London School of Economics and Political Science, wrote to the foundation criticising its website for being "riddled" with "multiple serious errors" about global temperature trends, citing a number of examples and inviting the foundation to urgently correct or explain them. Ward says he has yet to have received acknowledgement of his letter.

Responding to the publication of the foundation's accounts, Ward said: "We can now see that the campaign conducted by the Global Warming Policy Foundation, which includes lobbying newspaper editors and MPs, is well-funded by money from secret donors. Its income suggests that it only has about 80 members, which means that it is a fringe group promoting the interests of a very small number of politically motivated campaigners."

He continued: "We do not know whether the foundation's secret donors have vested interests in its campaign, which involves disseminating inaccurate and misleading information to the public and media about climate change, such as trends in global temperatures. This is outright hypocrisy from a group that constantly accuses climate researchers of not being open enough."