Ministers have just under a year to slash net migration by more than half after official figures showed another significant jump in immigration to the UK from within the European Union (EU).
Prime Minister David Cameron and Home Secretary Theresa May pledged to cut net migration - the difference between migrants leaving and arriving in the UK - to below 100,000 by the general election on May 7 next year.
But this target is moving further out of reach as new data from the Office for National Statistics (ONS) showed net migration increasing to 212,000 in the year to December, from 177,000 the previous year.
This was driven by a "statistically significant" increase in EU citizens immigrating to Britain, which the Government has little power to control, from 158,000 to 201,000 year-on-year.
As voters took to the polls in European and local elections, MPs and migration campaigners urged the Government to drop its " arbitrary " target.
Keith Vaz, chairman of the Home Affairs Select Committee, said: "Despite the Government's best efforts and repeated commitments to reducing the net migration figure it has remained the same.
"Controlling EU migration is impossible without reform. It is clear that their aim is unrealistic and unachievable. The Government should drop their target now."
Much of the focus during the elections will be on Nigel Farage's eurosceptic UK Independence Party (Ukip), which has campaigned almost solely on immigration issues.
Against a backdrop of growing concerns of an influx of Romanians and Bulgarians to the UK, and a surge in popularity for Ukip, the Prime Minister last year rushed through a series of tough measures to clamp down on "abuse" of free movement between EU member states.
Ukip leader Mr Farage said: "With under a year left to the general election the verdict is clear. David Cameron has broken his solemn promise to the British people on one of the most important political issues.
"And he has done so because he refuses to take back control of our borders in respect of more than 400 million people from more than two dozen countries on continental Europe."
Some 526,000 people immigrated to the UK in the year ending December, up from the 498,000 people who arrived during the previous year, while 314,000 emigrants left the country, down from the 321,000 the previous year.
Within these figures, 104,000 citizens of so-called EU15 countries - pre-2004 members of the union - arrived in the UK, up from 85,000 in the previous year.
Around 70,000 citizens of the EU8 countries, which joined the EU in 2004 and include Poland, came to the UK in the period, compared with 60,000 in the previous year.
And 23,000 citizens of Romanian and Bulgaria arrived in the period, before employment curbs for the two nations were lifted, up from 9,000 previous year.
Non-EU citizens immigrating to the UK dropped to 249,000 in the same period, from 260,000 the previous year.
Statisticians said this was due to fewer New Commonwealth citizens - such as those from India - arriving to study.
After the data was published, immigration and security minister James Brokenshire said: "Uncontrolled, mass immigration makes it difficult to maintain social cohesion, puts pressure on our public services and forces down wages for people on low incomes.
"While recent net migration levels remain stable the figures show that it has fallen by a third since its peak in 2005 under the last government and that this Government's reforms have cut net migration from outside the EU to levels not seen since the late 1990s.
"We are building an immigration system that is fair to British citizens and legitimate migrants and tough on those who abuse the system or flout the law. We want to ensure that people come to the UK for the right reasons - to work hard and contribute to our economy and society."
The number of Romanian and Bulgarians applying for National Insurance numbers increased to 65,000 in the year to March, from 35,00 the previous year, according to the new figures, but only 22% applied since restrictions to the labour market were lifted for the two countries on January 1.
Official employment figures released earlier this month actually showed a drop in the number of workers in the UK who were born in the eastern European countries.
Some 140,000 people who were born in Romania and Bulgaria were employed in the UK between January and March, a drop of 4,000 when compared with the 144,000 employed in the last three months of 2013.
Shadow home secretary Yvette Cooper said: "David Cameron and Theresa May's flagship immigration pledge is in shreds.
"They promised 'no ifs, no buts' to get their net migration target down to the tens of thousands by the end of the Parliament, and the actual figure is over twice that.
"By making such loud promises and failing to meet them, Theresa May and David Cameron have further eroded trust on immigration and undermined confidence in a government's ability to address people's concerns."
Sir Andrew Green, chairman of campaign group Migration Watch UK, said: "These new figures confirm that migration from the EU has blown the Government off course in meeting its overall target.
"In particular we note that new number of new National Insurance numbers issued to Romanian and Bulgarian citizens in the first quarter of this year was 45,000, of whom around 10,000 arrived in that quarter.
"This suggests that our estimate of a 50,000-a-year increase in the UK population of these citizens will be broadly correct."