Kim Hjelmgaard, and Gregg Zoroya

USA TODAY

LONDON — The United Kingdom's vote to leave the European Union last week appears to have been driven in part by a historical surge of immigrants in recent years that has transformed the island nation.

Legal migration, particularly from Eastern Europe, has soared as new countries have entered the EU and their citizens — with the blessing of U.K. leaders — took advantage of the bloc's freedom of labor laws to enter Britain.

Annual legal immigration into the U.K. is now 10 times what it was in 1993, and experts believe this cultural dynamic fed fears of globalization and job losses which, in turn, drove last Thursday's vote to exit the EU.

Opponents of a British exit — "Brexit" — from the 28-nation EU argue that these new residents often take more menial jobs rejected by the British-born.

"You have understandable concerns about the impact of immigration and the changing world we live in and what that means for pressure on services, schools and infrastructure," said Peter Catterall, a professor of sociology at the University of Westminster. "But on the other hand, the U.K. needs immigration for an aging work force in some areas and to produce a vibrant economy."

The migration influx has been so dramatic in recent years that currently one in 20 people living in the U.K. — 3 million people — were citizens of another EU country just two years ago, according to the British Office of National Statistics.

Before 2004, when the EU expanded to include 10 new member states such as Latvia, Poland and other eastern European nations, net EU immigration to the U.K. averaged around 10,000 per year, according to the national statistics office.

When that expansion occurred, the U.K., Ireland and Sweden allowed EU nationals to immigrate immediately under the bloc's freedom of labor laws. The move unleashed vast numbers relocating to Britain.

Last year, 270,000 citizens from EU countries immigrated to the U.K., the statistics office estimates. About 85,000 Britons left the island for other EU countries. The net migration to the U.K. of 185,000 was a record, according to Full Fact, an independent fact-checking organization.

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"We're such a small land mass. We can't assimilate people just pouring in with no limitation," said U.K. native Patricia Kawaja, who watched the voting unfold from her current home in Miami, Fla. and was sympathetic to native voting concerns about immigration. "Brits weren't flooding into Romania. It was always a one-way street to the strongest country."

Romania, along with Poland, are the two EU countries reporting the largest increase in immigrants to the U.K. in recent years, according to Full Fact. About 136,000 Romanians have relocated to Britain from 2011 to 2015.

A full third of those who voted in favor of Britain leaving the EU said immigration concerns were the reason. Fifty-three percent said they were motivated by the U.K.'s inability to make its own laws without interference from EU bureaucrats based in Brussels.

A research group predicted that the annual net migration to the U.K., if it were to remain in the bloc, would exceed 250,000 a year for the next 20 years. The estimate was by Migration Watch UK, which characterizes itself as independent and non-political but "concerned" about the scale of immigration to the U.K.

The "leave" camp used the issue of immigration to galvanize support for its cause. A week before the referendum, U.K. Independence Party leader Nigel Farage unveiled a campaign poster that showed long lines of migrants and refugees crossing borders. Farage was among the most high profile politicians advocating the U.K.'s exit from the European Union, and his party has gained popularity by promoting an anti-immigration theme.

After the "leave" camp's victory, opponents took to social media to complain that the ballot results had been fueled by xenophobia and even racism.

Boris Johnson, the former mayor of London and leave supporter who is now a potential candidate for prime minster, disputed that immigration was the driving factor in the referendum. He wrote in the Telegraph on Monday that, "after meeting thousands of people in the course of the campaign, I can tell you that the number one issue was control — a sense that British democracy was being undermined by the EU system, and we should restore to the people that vital power."

But troubling signs of anti-immigrant fervor have surfaced since the referendum. By Monday, there were reports of vandalism at a Polish community center and verbal threats and harassment of minorities. London Mayor Sadiq Khan asked city police to be on heightened alert to deal with an increase in hate crimes.

Zoroya reported from McLean, Va. Contributing: Jane Onyanga-Omara