Michael Douglas is one of a seemingly endless stream of celebrities promoting the cause of the United Nations.

Hundreds of stars, from screen icon Sophia Loren to erstwhile TV presenter Magenta Devine, have been seen spreading the word of the international peace organisation at photocalls throughout the world.

The UN first hit on the idea of celebrity ambassadors in the 1950s, when Hollywood and Broadway star Danny Kaye was taken on to promote children's rights.

He worked tirelessly throughout that year and subsequent decades. It was his success that led to the use of stars, like Sir Peter Ustinov and Audrey Hepburn, on a regular basis.

The current set of celebrities is led by a top flight of seven, called "Messengers of Peace", of which Michael Douglas is one.

Others include boxing legend Muhammad Ali, basketball star Magic Johnson and opera singer Luciano Pavarotti.

These are personally recruited by UN Secretary-General Kofi Annan. They must have a largely unblemished reputation, a proven interest in humanitarian issues and fame on a global scale.

The seven are expected to work hard - on a voluntary basis - to help focus attention on the work of the UN in general.

Most choose to focus on one particular area. Michael Douglas's main field, for example, is small arms control, while Muhammad Ali is a relentless campaigner for the poor.

But most other stars are recruited independently by the various UN agencies.

Thus the UN Development Program uses Brazilian footballer Ronaldo to highlight the plight of the poor.

Former Spice Girl Geri Halliwell has famously done her bit for the UN Population Fund, promoting safe sex to teenage girls in the Philippines.

And Unesco, the cultural, educational and scientific organisation, recently signed up 60-year-old Italian screen icon Claudia Cardinale, to join French actress Catherine Deneuve and opera singer Montserrat Caballe in promoting its causes.

Each agency has different "ranks" of celebrities who contribute varying amounts of time and effort.

For example Unicef, the UN children's fund, now has five Goodwill Ambassadors, including Sir Peter Ustinov, who represent it on an almost permanent and full-time basis.

Others become spokespeople for specific projects - Roger Moore, for example, is the major figurehead for the Unicef initiative to eliminate iodine deficiency disorders (IDD).

And there are those, like pop star Robbie Williams, who promote one-off projects or stage occasional benefit gigs.

There are no set procedures or contractual obligations on either side, although there is usually a symbolic swearing-in ceremony.

It is also important that the UN, with offices in more than 200 countries, is represented by an international mix of celebrities.

Many of their most important stars - such as Miss Universe Mpule Kwelagobe, recently appointed Goodwill Ambassador for Botswana for the UN Population Fund - may be almost unheard of in the UK.

On the whole, the UN says it is "very careful" to pick a celebrity with not only global fame, but longevity.

"We can't just have someone who's famous for five minutes," says a spokeswoman.

Similarly, they must also want to do more than just appear caring in front of the cameras.

"Our celebrities do it because they have real compassion for children," says the spokeswoman.

"They have the power to draw the cameras but they don't want the attention for themselves."

And they must be considered uncontroversial.

The Duchess of York almost landed a UN role - with the High Commissioner for Refugees - until the offer was swiftly dropped, amid rumours both of pressure from Buckingham Palace, and of the organisation's concerns about her "unsuitability".

Not all branches of the UN are convinced of the wisdom of using celebrities.

The UN High Commission for Refugees (UNHCR) decided to phase out formal international goodwill ambassadors, in favour of letting the stricken speak for themselves.

"Individual countries use goodwill ambassadors on an ad-hoc basis," says a spokeswoman.

"But we feel that the issues speak for themselves."