A "SUBSTANTIAL" number of eastern European migrants arriving in Britain in the last decade were not counted, statisticians have admitted.
Net migration in the UK between 2001 and 2011 was 346,000 higher than previously thought as a result of "inadequate sampling" of arrivals into the country, the Office for National Statistics (ONS) said.
Arrivals were not counted in part because an official survey focused on major airports, such as London Heathrow, London Gatwick and Manchester, while migrants were using an increasing number of regional routes into the country.
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Migration campaigners and experts said the disclosure underlined the struggle faced by the present Government to meet its target of slashing net migration to the tens of thousands by 2015.
Carlos Vargas Silva, senior researcher at the Migration Observatory at Oxford University, said: "We have known for some time that net migration must have been much higher during the 2001 to 2011 period than the official estimates had suggested.
"This report provides important evidence of the need for better migration data and of the limitations of using a survey to develop net migration data."
The ONS conducted a review of the quality of migration estimates, which are predominantly based on the International Passenger Survey (IPS), a system developed in the 1960s to collect information about passengers entering and leaving the UK.
People arriving from the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Poland, Slovakia and Slovenia were missed by the IPS between 2004 and 2008, the review found, as new routes into the country were added.
Routes connecting UK airports with the eight countries increased from 30 in 2001 to a peak of 190 in 2007, the ONS added, but the survey continued to focus on Britain's largest hubs.
The largest increases in routes and passenger journeys were at Luton and Stansted airports, with smaller increases recorded at Doncaster, Sheffield, Southampton and Bournemouth.
The ONS found IPS estimates of migrants under 15 were too low.
Sir Andrew Green, chairman and founder of campaigners Migration Watch, said: "This is final confirmation that net foreign migration under Labour totals nearly four million, two thirds from outside the European Union.
"It also shows that the peak of net migration was nearly 275,000 a year, making it even more difficult for the present Government to get the numbers down to tens of thousands."
The ONS's highest additional number of migrants was for 2006, when net migration is now thought to have been 67,000 higher than previously thought, reaching 265,000.