The United Nations Population Fund (UNFPA) says the number of people in the world has doubled since 1960.

World population will reach six billion during 1999, if it has not already done so. UNFPA is observing 12 October as "the day of six billion".

In a report, "The State of World Population 1999", UNFPA says there are some encouraging signs that human fertility is being reined in. UN statisticians now expect that the world's population in 2050 will be about 8.9 bn, not the 9.4 bn predicted as recently as three years ago.

Global fertility rates have declined faster than expected, and global life expectancy has grown from 46 years in 1950 to 66 today.

One in two under 25

But the picture is uneven, with very wide variations between countries and regions. In 61 countries, containing about 44% of world numbers, couples are having fewer than the two children needed to keep the population stable.

UNFPA's goodwill ambassador Geri Halliwell has tried to promote safe sex and greater use of condoms

Yet, as the report puts it, "half the world is under 25, and there are over 1 bn young people between 15 and 24, the parents of the next generation".

The report says this "population momentum" will account for up to two-thirds of the projected growth of world population - now rising by 78 m people annually.

In 1969, world population was growing at 2.4% a year. Today the rate is 1.3%. But most of the growth is in south and west Asia and in sub-Saharan Africa, poor regions already coping with severe demographic and environmental stresses.

Life expectancy plummets

"Over a billion people are still deprived of basic needs. Of the 4.8 bn people in developing countries, nearly three-fifths lack basic sanitation. Almost a third have no access to clean water. A quarter do not have adequate housing, and a fifth have no access to modern health services.

"The cumulative effects of continuing poverty, gender discrimination, HIV/AIDS, environmental change and shrinking resources for development have the potential to wipe out the benefits of lower birth rates".

HIV/Aids, the report says, is responsible for about a third of the reduction in long-term population projections, and it is spreading faster than expected.

"In Botswana, where one of every four adults is infected, life expectancy has fallen from 61 years in the late 1980s to 47 today, and is expected to plunge to 38 by 2005-2010. Nevertheless, the population is still expected to nearly double by 2050."