The government's immigration curbs will not be enough to achieve David Cameron's target of cutting net migration to the "tens of thousands", a think-tank warned today.

The best hope of fulfilling the Prime Minister's pledge was for an economic downturn to make the country less attractive to migrants and drive away EU migrants already here, the Institute for Public Policy Research (IPPR) said.

Its predictions for 2012 showed net migration - the difference between the number of people entering the UK and the number leaving - falling from a record 252,000 in 2010 to 180,000 this year.

Matt Cavanagh, the think-tank's associate director for UK migration, said: "While policy changes will start to achieve significant reductions in immigration from outside the EU, this will not be enough to put the Government on track to hit its target."

Restrictions on immigration brought in by the government could also be a drag on the UK economy when the demand for more workers picks up, he added.

Overall, 2012 "will be a crucial year for coalition ministers", Mr Cavanagh said.

While there was strong public support for reducing immigration, there were also "surveys showing equally strong doubts that they will deliver this, and very mixed support at the level of detailed policies".

"By promising what it cannot deliver, the government, far from achieving its stated aim of taking the heat out of this emotive issue, will instead feed the public's sense of disillusionment," he said.

The IPPR predicted that the number of migrants coming to the UK from outside the EU would fall by about 10% in 2012, fuelled by new restrictions on foreign students and worsening economic conditions.

The number of EU migrants coming to the UK could also fall, and the number leaving rise, as the UK's economic prospects decline relative to EU countries such as Poland, now the largest immigrant group by nationality in the UK.

But the IPPR said further curbs on skilled migrants coming to the UK, such as a higher salary threshold for those on intra-company transfers, were unlikely to reduce overall numbers by more than 10,000.

Further restrictions on family migration were also likely to have little immediate effect as they are expected to be held up by legal challenges in the courts, the report said.

And it predicted that figures published early this year will show the number of foreign prisoners who have been returned to their home countries during 2011 will have fallen, piling pressure on the UK Border Agency (UKBA) when it is already faced with significant staff cuts.

Immigration Minister Damian Green said: "IPPR's predicted reduction in net migration of 70,000 by the end of 2012 is consistent with hitting our target of reducing net migration to the tens of thousands by the end of this parliament.

"We've limited non-EU workers coming to the UK, overhauled the student visa system and will shortly announce reforms of the family migration and settlement routes.

"The latest quarterly figures show that student visas issued are down 13% and the main work visas issued are down 18% compared to last year - an early sign that our policies are starting to take effect."