Several hundred thousand more female immigrants than had been expected surged into Britain over the past decade, boosting not only the numbers of foreign-born residents but also leading to a mini-baby boom, an analysis of 2011 census data shows.

Data released by the Office for National Statistics in July showed that net migration – the number of new migrants minus the number moving abroad – to England and Wales increased the population by more than 2.1m.

However, closer analysis shows that when the ONS tried to forecast immigration figures the year before the census, it underestimated the number of female immigrants by 361,000, and overestimated the number of male immigrants by 94,000. Moreover, the surge in female migrants has been concentrated in the 20- to 44-year-old age bracket, the prime years for bearing children.

Official data show that more than a quarter of all births are to non-UK born women, which has boosted the population aged 0 to 8 years by nearly 300,000 since the last census in 2001. Polish women overtook those from Pakistan as the single largest group of foreign-born mothers in 2010 and 2011, accounting for more than 10 per cent of births within this group.

The latest data come as immigration is rising to the top of the political agenda. Prime Minister David Cameron this month announced a review into the freedom of EU migrants to live and work in the UK– although limitations are barred under EU agreements – responding to concerns that immigrants are taking jobs that young unemployed Britons cannot obtain.

But economists and demographers have a more positive view of immigration, as UK fertility rates are too low to maintain even population levels. As longevity increases, the burden of paying for the nation’s elderly falls on fewer and fewer shoulders.

ONS data released in August show that UK-born women are having an average of 1.9 children each, below the rate of 2.1 that is considered necessary to keep a population level. But when non-UK born women are included, the rate rises to around 2.0.

Gordon Sharp, head of the actuarial profession’s Continuous Mortality Investigations unit which studies population changes, said the influx of women of child-bearing age and the resultant “mini-baby boom” was likely to be a good thing for the UK’s long term finances.

Even if the state pension age does not rise further than is planned, he said, the burden of supporting the nation’s elderly in 30 years’ time would nevertheless be spread among more workers.

“It means the dependency ratio is not as severe as it might otherwise be,” Mr Sharp said. However, he noted that there were also near term fiscal costs, including higher expenditure on healthcare and education.

Professor John Salt, head of University College London’s migration research unit, said that the most likely cause for incorrect forecasts of female immigration was the fact that earlier migration was heavily male. Those male migrants who did not return to their own countries when the recession hit in 2008 were likely to have encouraged their partners to move to the UK as well, he suggested.

“You’ve got the creation of a big enough [immigrant] population here and, before you know it, you’ve got a marriage market,” he said.

Although immigration has the general effect of raising the rate at which an economy grows, population experts warn that increased migration was not a long-term fix for declining fertility rates. Jean-Christophe Dumont, head of the international migration division at the Organisation for Economic Cooperation and Development, noted that eventually those immigrants will grow old and turn to their host country’s social security system for support, requiring greater numbers of new immigrants.

“It should be clear that migration cannot offset the long-term demographic trend,” he said.

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