Annual net migration to Britain remains at a record high of more than 250,000 a year despite the declared ambition of the home secretary, Theresa May, to reduce it below 100,000 by the time of the next election, according to figures published on Thursday.

The Office for National Statistics (ONS) said the politically sensitive figure for net migration – the number of people coming to stay in the UK for longer than 12 months minus the number going to live abroad for more than 12 months – in the year to September 2011 was 252,000.

This is only 3,000 below the 255,000 recorded in the previous year to September 2010 and is a rise of 2,000 from the previous quarter's figure of 250,000 in the year to June 2011.

The latest figures are a blow to Home Office ministers, who have put in place new curbs on non-European workers, including an annual cap on skilled migrants and a clampdown on overseas students but have yet to see the net migration figures moving in the direction of their target.

Downing Street said the government remained committed to getting net migration down to "tens of thousands".

"There is evidence that the changes we are making to the rules are starting to have an impact. It is still our intention to bring the levels of net migration back down to the tens of thousands. Clearly it is going to take some time," said the prime minister's official spokesman.

New moves to curb the numbers coming to join close family members and spouses by as much as two-thirds are also expected to be detailed shortly.

The ONS said that long-term immigration to the UK remained steady at 589,000 – a level it has remained at since 2004. An estimated 252,000 people left Britain to the join the 5 million British citizens already living abroad. About 190,000 people left the UK to work abroad.

Studying remains the most common reason for coming to live in Britain with 250,000 overseas students arriving in the year to September 2011 – a similar level to the previous year.

The Home Office, however, pointed to the "leading indicator" of more recent data on the number of student visas issued rather than the ONS estimates of student migration which show a 21% fall in overseas students coming to Britain in the year to March 2012. Ministers claim that an apparent 62% fall in the number of student visas between January and March this year shows that a recent drive to curb their numbers is working.

The figures also show that the number of people applying for asylum fell slightly to 4,844 in the first three months of 2012, with 706 applicants from Pakistan and 575 from Iran.

The number of people detained for immigration reasons in Britain, which is already the highest in Europe, reached new record levels with 7,516 incarcerated in the first three months of this year – an increase of 7%. They included 53 children, of whom 35 were detained in the new "family-friendly" Cedars pre-departure unit near Gatwick airport. The coalition pledged to put an end to child detention.

Ministers will be concerned to see that removals and deportations fell again during the first three months of this year to 13,386, a 6% drop compared with the same period in 2011 and more than 25% down from a peak in 2008 when a major effort was made to deport jailed foreign nationals.

The number of people granted settlement in Britain in the year to March 2012 fell by 35% from 226,478 to 148,144 but this was mainly because of a backlog of long-term asylum "legacy" cases moving out of the system. The number of new British citizens also fell by 10% to 175,298.

The immigration minister, Damian Green, said the figures show signs of progress: "Our tough new rules are now making a real difference with a record 62% drop in student visas in the first quarter of 2012, and overall falls in work visas, family numbers and people settling.

"As these policies start to bite we are seeing an end to the years when net migration was consistently on the rise. But the hangover from the old system of weak controls means it is still too high and we will continue our programme of reforms to bring net migration down from the hundreds of thousands to the tens of thousands."

However, the Migration Watch UK chairman, Sir Andrew Green, who campaigns for zero net migration, said "tough measures" were now needed: "You cannot expect to repair 15 years of mismanagement in 15 months, but it is still worrying news that net migration is running at a quarter of a million a year. There is no sign of any reduction from the huge numbers that developed under Labour."

Yvette Cooper MP, the shadow home secretary, said the government was failing badly on its own immigration target to reduce net migration: "David Cameron said 12 months ago: 'No ifs. No buts. That's a promise we made to the British people.' Yet these figures show that promise is being broken and the net migration figure of 252,000 is barely changed in the last two years, even with the net rise in British citizens leaving the UK.

"Migration experts have said the government does not have the policies to deliver on the target, so the prime minister is not being straight with the public on immigration. He should not make promises he can't keep on such an important policy area," said Cooper.