Geri Halliwell in New York speaks about her new role

Her role for the UN's population fund will include promoting the organisation's programme on contraception and Aids prevention around Britain.

Speaking in New York about her new role, the singer pledged to publicise the plight of millions of women denied basic human rights.

Danny Kaye: Promoted children's rights

Halliwell, who recently signed a new record deal, is to be the UK ambassador for the population fund and will work with the Marie Stopes charity on issues including sexual health.

She spoke of her "delight" at being able to speak out on behalf of women in developing countries who are denied what she described as their rights over reproductive health care and family planning.

She said: "This is a tremendous honour for me and I am looking forward to being involved in this important project.

"Like many women, I take my right to make reproductive and sexual choices for granted but for women in the developing world, who have few real choices in this area, getting pregnant can mean the difference between life and death."

The star said she would not be content to take a back seat in her new role, saying: "I want to put my face to this but I cannot just do that. I have to get my hands dirty.

"I am going to really, really try and learn a lot about this and I hope the world will learn with me.

The former Spice Girl's first role will be to support an essay competition called 6 Billion and One - so named because the world population is about to reach six billion.

Geri arrives in New York ahead of her meeting with Kofi Annan

The contest asks schoolchildren aged 16-18 to write to Tony Blair with the aim of getting the British Government's support for international action to guarantee reproductive and sexual health rights for all.

Speaking at the UN in New York, Miss Halliwell said: "British teenagers need to speak out about these issues, because the actions we take now will have a profound effect on the world which they and their children will inherit."

The former Spice Girl is the latest in a long line of celebrity diplomats - the first, the Hollywood and Broadway star Danny Kaye, was such a success in promoting the rights of children that the tradition has grown from strength to strength.

Film legend Audrey Hepburn travelled to Somalia during the country's darkest hour and the UN is now calling on celebrities such as Mohammad Ali, Michael Douglas and Luciano Pavarotti to help promote its causes.

Busy with other charity work

Halliwell has been busy with other charity work since she left the Spice Girls in May.

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In September she agreed to front a breast cancer awareness campaign, and spoke of the breast cancer scare that she went through when she was 18.

Audrey Hepburn comforting a child in Somalia

Last week she travelled to Uganda to make a film for Comic Relief.

But as Halliwell begins her UN role, she has a long way to go before she matches the example of actor Sir Peter Ustinov.

In August, the UN's Secretary-General paid tribute to Sir Peter's 30 years of working with the UN.

"His qualifictions are unbeatable," Mr Annan said. "He speaks French, German, English, Italian, Russian, Spanish and even some Turkish and Greek. He could double as a representative of all the member states."