He said he would campaign for nuclear disarmament, the control of small arms and payment of the huge US debt to the world body.

"This probably means as much to me as any of the two Oscars that I got," he told a news conference on Thursday, after an introduction by UN Secretary-General Kofi Annan.

Douglas won his first Academy Award in 1975 for producing One Flew Over the Cuckoo's Nest, and a best actor Oscar in 1987 for Wall Street.

He plans to use his celebrity status to call for the elimination of nuclear weapons and non-proliferation of small arms.

The actor said he became interested in the nuclear issue after starring in 1978 movie The China Syndrome.

He also said he was "deeply embarrassed about our debt to the United Nations as a US citizen". The United States is the biggest UN contributor, but also the largest debtor, owing about $1.5bn to the organisation.

Koffi Annan has already signed up four other stars from the world of entertainment and sport: singer Enrico Macias, opera star Luciano Pavarotti, writer Elie Wiesel and basketball player Earvin "Magic" Johnson as "messengers of peace."

All the world's a stage...

Douglas is one of a long line of celebrities to step out onto the world stage of international politics.

• Mick Jagger's ex-wife Bianca is human rights campaigner for Amnesty International. This week she told British MPs in the House of Commons of the horrors she had witnessed in Bosnia and more recently in war-torn Kosovo. She has called on Nato to use force to stop Serb violence against ethnic Albanians in the region.

• Actor Richard Gere is a Buddhist and staunch supporter of the Tibetan independence movement. He has founded his own charity, the Gere Foundation to promote awareness of China's aggressive hold on the tiny Himalayan state in 1949.

• Actress Jane Fonda angered Americans in the Seventies by speaking out against the US government over Vietnam. She made several speeches in Hanoi and became known as Hanoi Jane. More recently Fonda created a stir by supporting a "Truth for Youth" education drive for safe sex backed by British condom manufacturer Durex.