Image caption Fertility must drop below replacement level in order to stabilise the world population says a UN report.

The world population growth rate must slow down significantly to avoid reaching unsustainable levels, says a new UN report.

To have a reasonable chance of stabilising world population, fertility must drop to below "replacement level".

It must then be maintained at that level for an extended period, says the report.

This replacement level is the fertility level at which a population replaces itself from one generation to the next.

The world population is already poised to reach 7 billion later this year and this figure potentially could double to 14 billion by 2100 if action is not taken.

Even countries with intermediate fertility need to reduce it to replacement level or below if they wish to avoid reaching unsustainable population levels Hania Zlotnik, Director, UN Population Division

This is of particular concern for the least developed countries worldwide, which are growing at the fastest rate and are already the most vulnerable to famine.

The UN Population Division have produced six projections of potential future population change based on different changes to fertility level and other factors.

In the medium scenario, world population peaks at 9.4 billion in 2070 and then starts to decline.

However for this to happen, fertility needs to decline significantly in most developing countries.

No guarantee

In recent years, there has been widespread acceptance of the medium scenario as almost a certainty.

However Hania Zlotnik, the Director of the UN Population Division says there is "no guarantee that this scenario will become a reality because high-fertility countries may not reduce their fertility fast enough and countries with intermediate fertility levels may see them stagnate above replacement level".

"Even countries with intermediate fertility need to reduce it to replacement level or below if they wish to avert continuous population increases to unsustainable levels."

Replacement level is the level of fertility at which a population exactly replaces itself from one generation to the next.

In high mortality countries it is 2.5 children per woman and in low mortality countries it is 2.1 children per woman.

The high and low projections reveal how even relatively small deviations from replacement-level fertility can lead to dramatic changes in the size of the world population.

The high scenario, where fertility remains mostly between 2.2 and 2.3 children per woman, would lead to a world population of nearly 30 billion in 2300.

The report says that "even with significant fertility reductions, Africa's population will likely increase by 150% by 2100 and many of its countries will see their populations increase four-fold or more".

It warns that although the reduction of fertility may be inevitable, considerable effort over the next few decades is required to make it a reality.

The "World Demographic Trends" report has been released by the UN Population Division today ahead of the UN Commission on Population and Development.