Czech president condemns 'aggressive' Heartland Institute adverts.

Václav Klaus, the Czech president and prominent climate sceptic, has condemned a controversial billboard campaign used by a rightwing US thinktank to advertise the forthcoming conference at which he is scheduled to give the keynote speech. However, his spokesman said Klaus will not join other speakers who have pulled out in protest and says he still intends to proceed with the engagement.

Last week, the Heartland Institute, a thinktank based in Chicago that promotes climate scepticism, withdrew an advertising campaign that compared people concerned about climate change with mass murderers and terrorists such as Osama bin Laden, Charles Manson and Ted Kaczynski. Despite later removing a billboard which appeared over an Illinois expressway featuring a picture of Kaczynski with the caption: "I still believe in global warming. Do you?", the group refused to apologise for the campaign, instead describing it as an "experiment".

Klaus, who is scheduled to give the keynote speech at the Chicago Hilton on 21 May, has previously appeared at the thinktank's annual conference for climate sceptics, as well as given keynotes for similar climate sceptic groups such as Lord Lawson's Global Warming Policy Foundation in the UK and the Institute of Public Affairs in Australia.

Klaus's spokesman told the Guardian: "President Klaus is principally against non-serious, aggressive and provocative billboard campaigns of the kind the Heartland Institute used recently." Asked if Klaus would now be pulling out, his spokesman said: "Mr president will attend this conference."

Ross McKitrick, a Canadian economist, and Donna Laframboise, a Canadian climate sceptic blogger, have both confirmed that they will no longer speak at the conference in protest at the billboard and Heartland's continued refusal to apologise.

The controversy has led to a wave of corporate donors publicly stating that they will no longer financially support the thinktank. On Wednesday, the United Services Automobile Association, an insurance and financial services company for US military families, confirmed on its Facebook page it was the latest donor to abandon Heartland. It joined other insurance companies, such as State Farm, and the drinks giant Diageo in cutting ties with the group. The Sierra Club, one of the US's leading conservation groups, said on Wednesday 20,000 people had backed its call for corporations to pull the plug on Heartland.