The working population is projected to rise over the coming decades

The Office for National Statistics says an analysis of population and migration projections reveal a long-term reversal in falling fertility.

Previously published figures show two thirds of recent population rises are down to migration factors.

The ONS says the UK is witnessing the fastest growth in population since the 1960s and part of the jump is due to rising fertility among all women.

In October, the ONS predicted the UK population could reach 71 million by 2031, with migrants and their UK-born children accounting for 69% of that growth.

CHANGES IN FERTILITY 2002-06 UK-born women: 1.5 to 1.7 children Non-UK born women: 2.3 to 2.5 Source: ONS

And in what it says is its first annual article on the population, ONS experts have focused on the role of fertility within those projections.

According to its figures, both UK-born and foreign-born women are contributing to a "consistent rise" in fertility which began after 2001.

Total fertility in the UK has risen from 1.6 children per woman in 2001 to 1.8 - the highest level since 1980. Within that, there are now 1.7 births per UK-born woman and 2.5 births per foreign-born woman.

Analysis indicates that women born both in the UK or overseas have contributed to rising fertility by having babies in their late twenties or thirties.

This indicates a demographic shift as women have chosen to delay having children until they are older for career reasons.

Women born overseas now account for a fifth of all births in the UK, some 154,000 in 2006.

UK 'unusual'

But National Statistician Karen Dunnell said the analysis showed that women from all backgrounds were contributing to population growth through higher fertility.

POPULATION PROJECTIONS 2031: 71m (probable) 2031: 75m (high) 2081: 85m (probable) 2081: 108m (high) Source: ONS UK population may double by 2081

"The UK is unusual in having a rising fertility rate," she said. "All countries in the western world are ageing because death rates are coming down.

"Many countries have very high levels of in and out migration like the UK. In those ways we are fairly typical."

The annual report says that the UK's population has been growing at its fastest rate in forty years and that it is also ageing.

According to her department's projections, there could be almost 3m people over 85-years-old by 2031 - more that double the number today and a demographic change with major implications for public spending.

The ONS has been reviewing how it presents data on migration and population amid political controversy over the reliability of currently available statistics.

The body is expected to start publishing annual reports on migration alongside more detailed quarterly statements.

Experts say they are trying to work out ways of providing local areas with more up-to-date information on demographic changes.