ONS: Immigration into Britain in the 00s was nearly 350,000 higher than previously thought

The ONS blamed the error on the “inadequate sampling” of passengers (Getty Images)

Net migration into the UK during the previous decade was 346,000 higher than previously claimed after officials failed to properly count new arrivals landing at regional airports, according to new figures from the Office for National Statistics.

The embarrassing miscalculation reveals an influx equivalent to the population of Cardiff and has fuelled persistent claims that immigration from Eastern Europe under Labour was far higher than earlier official estimates suggested.

The ONS blamed the error on the “inadequate sampling” of passengers. Statisticians focused too heavily on counting migrants touching down at the major airports of Heathrow, Gatwick and Manchester.

This was despite the budget air revolution of the post-millennium decade which had witnessed spectacular growth in the number of low-cost carriers setting up new routes.

Many of these were from new European Union countries and connected with airports such as Luton, Stansted as well as Doncaster, Sheffield, Bournemouth and Southampton. Regional hubs witnessed dramatic increase in passenger numbers in the first half of the decade when it was estimated four out of 10 arrivals touched down at a local airport.

A spokesman for David Cameron said the Prime Minister had full confidence in the work of the ONS which compiles official data for Government policymakers.

Mr Cameron who is under mounting pressure to curb rising numbers of migrants amid a growing challenge from the UK Independence Party and within his own party, has pledged to reduce the net numbers of new arrivals to below 100,000 by 2015. The latest figures suggest the task could be more difficult than thought.

“What the latest ONS statistics underline is the point at which, in the 2000s, immigration was out of control,” a Downing Street spokesman said.

The figures examined the number of migrants coming to the UK between 2001 and 2011. The original figures were based on the International Passenger Survey (IPS) originally developed in the 1960s to track population movements in and out of the UK.

However. the number of routes connecting UK airports with Czech Republic, Estonia, Hungary, Latvia, Lithuania, Poland, Slovakia and Slovenia – which acceded to the EU in 2004 - increased from 30 in 2001 to a peak of 190 in 2007. This meant citizens arriving at regional airports from these countries in the four years from 2004-08 were not counted in the official data.

The ONS also found estimates of migrant children under 15 arriving at the major airports were too low.

Carlos Vargas Silva, a 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."

Under the new counting system immigration into the UK peaked in 2006 at 265,000.when the number of arrivals was 67,000 higher than suggested by the original estimates. It had previously been thought that net migration was at its highest in 2010 when the figure was 252,000. Last year net migration increased to 212,000 – up from 154,000 the previous year.

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."