Women tend to have children later

It could lead to a fall of almost a quarter - or 88 million people - by the year 2100, they say.

The continent's population is continuing to grow at the moment they say, but it will enter a "negative momentum" due to the declining birth rate.

The researchers say the turning point came in 2000, when the number of children dropped to a level that meant there would be fewer parents in the next generation than there are in the current one.

There are now more adults than children in the European population.

Researchers say the trend is likely to put pressure on the social security and retirement provision in the future.

Population data from 15 countries across Europe was studied to determine the potential impact of child-bearing patterns.

The researchers say around 40% of the projected population decline could be attributable to women's decision to put off having children.

Pressure

On average, women in Europe now have a fertility rate of 1.5 births each.

If this continues until 2020, they say it could lead to at least 88m fewer people living in the EU in 2100, assuming constant levels of mortality and no significant effects of migration.

In 2000, the EU population was around 375m, so this would mean a fall of more than 23%.

Wolfgang Lutz of Vienna's Academy of Sciences, who led the research, said a steadily dropping birth rate and an increase in average age of the population will put pressure on Europe's social security and retirement systems and could lead to a slowdown in productivity.

Brian O'Neill of Austria's International Institute for Applied Systems, who was also involved in the research, said: "We've found that the timing of child-bearing can actually have a major impact on future demographic trends."

He added: "Age structure affects population size because it determines how many adults there will be in the future.

"Currently there are fewer children than adults. That can produce a declining population in the future because the number of potential parents will be smaller than the number of parents there are today."

Migration

But he said other factors would come into play.

"That is going to push the population toward decline. Whether it actually ends up in a decline will depend on other factors, particularly migration.

"The European population may not decline if migration is high enough to offset this population momentum."