(MADRID) - Soaring numbers of young immigrants have compensated for a low birthrate in Spain in recent years, but the country is still expected to have one of the oldest populations in the world by the middle of the century, experts say.

The arrival of millions of young foreigners, mainly from Latin America, has lowered the average age of the population, said Carmen Gonzalez Enriquez, a researcher for the Elcano Institute, a public policy research organization.

The immigrants, who now make up around 10 percent of Spain's 45 million residents, have also pushed up the country's average birthrate.

"Today, a quarter of births are from immigrant families," she said.

Spain's birthrate has gone from 1.2 per woman in 1996 to 1.4 in 2006, according to the European Union's statistical agency Eurostat.

But it is still far from ensuring that the size of the population remains steady, which requires a rate of 2.1 babies per woman of childbearing age.

The low birthrate results from the lack of any serious policy to encourage Spanish couples to have more children since the return of democracy in the 1970s, said economist Josep Oliver.

He said special allowances for new babies remind many people of the 1939-75 dictatorship of Francisco Franco, and the high unemployment rate among young people in the 1980s and 90s meant the government was unwilling to encourage them to have children.

At the start of this century, Spain had a "real shock" when it discovered that it lacked the workers needed to respond to an economic boom, Oliver said.

As a result, Prime Minister Jose Luis Rodriguez Zapatero declared an amnesty in 2005 for 580,000 illegal immigrants.

The measure helped provide the workers the country needed, but also responded to the "huge fall in the population," Oliver said.

As they became legal, these foreigners began paying into the social security system, to which they now contribute 8.0 billion euros (12.8 billion dollars.

Today the immigrants, who are mostly young and healthy, are not a burden on the social security system. But that will not be the case when they reach retirement age, at a time when baby boomers born in the 1960s and 70s also end their working lives, warned Gonzalez Enriquez.

Immigration has also become a major political issue.

During the campaign for legislative elections in March, conservative opposition leader Mariano Rajoy raised the spectre of a massive influx of immigrants to a country "where there is no more room."

But Oliver said "we are going to need immigrants for many years" faced with an aging society.

The Paris-based Organisation for Economic Cooperation and Development (OECD) has forecast that by 2050 Spain will have the third oldest population, behind Japan and South Korea.

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