Spain took in an extra 650,000 foreign immigrants last year, mostly from Latin America although Britons heading for a life in the sun also accounted for a significant proportion.
Booming Spain has been Europe's largest absorber of migrants for the past six years, with its immigrant population increasing fourfold as 2.8 million people have arrived.
Spain was a country with net emigration until the late 1980s. Now, after 10 years of economic growth, it has 3.7 million immigrants of its own - or 8.7% of the population. "The reality of the situation overwhelms the provisions of desk-bound sociologists," El País newspaper commented today.
The new arrivals have mostly been absorbed into Spain's growing economy, although many have joined the plentiful supply of illegal, underpaid labour.
Some 47,000 Britons, equivalent to the entire population of a town the size of Kettering or Hereford, signed on as new residents at Spanish town halls last year. That brought the registered British population in Spain to 274,000 - equivalent to a city the size of Bradford or Leicester.
But experts estimate that up to three times as many Britons, about 750,000 people, spend a significant part of the year living in Spain. Only Morocco, Ecuador and Romania have more foreign residents in Spain. Germany, which comes second among the EU countries, provides only half as many.
Two Spanish provinces - Alicante and Málaga - account for more than 50% of British residents. The fastest-growing area, however, is the south-eastern province of Murcia - where new resorts are springing up in one of the driest corners of Spain. The British population of the province increased by more than a third to 13,300 last year.
Spectacular growth in Spain's immigrant population comes as the country's economy has created more than half of all the new jobs in the European Union over the past five years.
Although Spain's transformation from a country of emigrants to a country of immigrants has not yet provoked major social tensions there is concern that this may be on its way.
"We may not be able to cope with the entry of so many immigrants in so little time," says Rafael Pampillón, the head of the economics department at Madrid's Instituto de Empresa business school.
At least 700,000 immigrants from Latin America, eastern Europe and north Africa live in Spain without proper residency or work permits, government officials have said. The figure is down by 650,000 since last year, when the Socialist government decreed an immigration amnesty.
It was the sixth immigration amnesty in Spain in 16 years, and was predicted to increase social security contributions by up to £1bn a year.
Opposition politicians said there may be a million more foreigners living illegally in the country. "This is what we call the siren affect," said Ana Pastor, spokeswoman for the conservative People's party, which says last year's amnesty simply encouraged more illegal immigrants.
Although sub-Saharan Africans barely show on the official statistics, west African immigrants are arriving by their hundreds on rickety boats bound for the Canary Islands.
Economists say immigrant labour has been crucial to Spain's 10-year economic boom, keeping down wage costs and providing cheap domestic help for working women. Increased social security contributions have helped push a looming pensions crisis further into the distance, while young immigrant families have finally turned around a birth-rate that had become one of the world's lowest.