Research showing more than five million people were granted citizenship in the European Union over five years shows how "little control" Britain has over its borders, Leave campaigners have claimed.
Around 890,000 people became citizens in one of the bloc's 28 countries in 2014, including 125,605 in the UK, according to official figures.
Moroccans made up the largest group of beneficiaries overall, followed by Albanians and Turks, according to the Eurostat bulletin.
Around 1.1 million citizenships were granted by the UK over five years but the number dropped from 203,000 in 2009 to 125,000 in 2014.
Freedom of movement rules mean the nearly four million new citizens in the rest of the EU now have the right to come to Britain, Vote Leave said.
Chief executive Matthew Elliott said: "These figures show how little control we have over our borders while we stay in the EU.
"Since 2009, close to four million people have been given the right to come to the UK, with Moroccans, Albanians and Turkish citizens the main beneficiaries. The number of people coming to the EU is increasing each year and it is becoming unsustainable.
"This level of migration puts a strain on our invaluable public services, as well as jobs and wages. The British people are absolutely right to be concerned."
Around nine in 10 of the people granted citizenship in the 2014 were from countries outside the EU.
Some 92,700 Moroccans gained the new status, 88% of them in Spain, Italy or France, along with 41,000 Albanians, nearly all in Greece or Italy, and 37,500 Turks, 60% of whom were in Germany.
Almost two thirds of the 35,300 people from India who acquired citizenship did so in Britain, according to the research.
Ukip migration spokesman Steven Woolfe said: "Every one of these 900,000 people granted EU citizenship now have an untrammelled right to come to live in the UK because of our EU membership. And this process will continue into the future unless we vote Leave on June 23rd.
"By voting to leave, we will take back control of our immigration policy, half of which is completely under EU command.
"By doing this we can enhance our security and increase the wages of ordinary working people, many of whom have seen their wages depressed by an oversupply of cheap labour from Eastern Europe."
A spokesman for Britain Stronger In Europe said: "The overwhelming majority of independent experts agree that leaving the EU would damage the economy - pushing up prices, risking jobs and reducing the amount of money available for public services such as the NHS.
"Throwing our economy into this kind of uncertainty is not the answer to immigration, an issue that is much more complex than the Leave campaign would have the public believe.
"The Leave campaign is failing to paint a consistent picture of life outside the EU. It is clear that leaving Europe is too big a risk to take."
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