The Government has insisted it remains committed to cutting net migration to Britain to the "tens of thousands" - despite it hitting a record high last year.
Official figures from the Office for National Statistics (ONS) showed long-term net migration in 2010 was 252,000 - the highest calendar year total on record.
The figures represent a big increase on the 2009 total of 198,000 - although ministers said they had now peaked and were starting to come down.
Downing Street said David Cameron still believed he could meet his target to get net migration down below the 100,000-a-year mark by the end of the Parliament.
The Prime Minister's official spokesman added: "Clearly that process is going to take some time."
However, Oxford University's Migration Observatory warned ministers would have to slash immigration from outside the European Union by 70% if they were to achieve the target.
The ONS said that while immigration had been broadly steady at 591,000 last year, there had been a sharp fall in emigration with 339,000 leaving the country - the lowest total since 2001.
While net migration peaked at 255,000 in the 12 months to September 2010, provisional figures for the year ending March 2011 showed it had fallen back to 245,000.
Nevertheless, the figures remain at historically high levels. The previous record for a calendar year was 245,000 in 2004 - although mid-year data for the 12 months to June 2005 reached 260,000.
Immigration minister Damian Green said overall the figures presented an "encouraging" picture, although he acknowledged there was a lot still to be done to meet the target.
"These figures show that the Government was right to take swift action to overhaul the immigration system," he said.
"Latest quarterly figures show a decrease in the number of student and work visas issued compared to a year earlier - an early sign that our policies are starting to take effect.
"The latest net migration figures are also encouraging, showing a fall since the recent peak in September 2010, but we are clear there is much more to be done."
The Migration Observatory said the figures included 34,000 net migration of EU nationals which the Government could not restrict.
Therefore, the entire 152,000 cut in the total that would be needed to hit the target would have to come from the remaining 218,000 net migration from outside the EU - a 70% reduction.
"This is not the news that the Government wanted to hear," said the Migration Observatory's director, Martin Ruhs.
Meanwhile, Home Office figures show a sharp drop in the numbers of failed asylum seekers and other illegal migrants being removed from the country.
In the three months to September 2011, 13,253 were removed or departed voluntarily - a 13% drop on the 15,261 who left in the same period last year.
At the same time, asylum applications were up 9% - from 4,276 in the third quarter of last year to 4,912. The main sources of increase were claimants coming from Pakistan, Iran and Syria.
Labour pointed to a 12% drop in the number of non-asylum seekers who were refused entry in the three months to September 2011 - the period covered by Home Secretary Theresa May's controversial relaxation of border controls - from 4,730 to 4,141.
Shadow immigration minister Chris Bryant said the figures called into question Mrs May's claims that the pilot programme had been a success.
"Far from improving the number of detections of people with criminal records or not wanted in this country for other reasons, these figures suggest fewer of those people were stopped," he said.
However, Mr Green said Labour's claims were based on "a misunderstanding of both the immigration system and the immigration statistics".
A Home Office spokesman said the relaxation had only covered nationals of the European Economic Area (EEA).
The number of passengers with EEA passports who had been stopped at the border had risen from 108 in the third quarter of 2010 to 144 in the same period this year - a 33% increase.