Prime Minister David Cameron and Home Secretary Theresa May want to bring annual net migration down from the current 242,000 to tens of thousands of people by 2015.
But they will only achieve half that figure at best, according to analysis by Oxford University's Migration Observatory.
Reforms to the immigration system will not help reduce numbers far enough or fast enough, leaving net migration at around 165,000 by 2015, the report said.
Mr Cameron admitted yesterday that the Liberal Democrats were preventing the Tories from taking tougher action on immigration.
"We've all had to make compromises," he told BBC Radio 2's Steve Wright in the Afternoon.
"If I was running a Conservative-only Government I think we would be making further steps on things like immigration control or making sure that our welfare reforms were absolutely making sure that if you're not prepared to work you can't go on welfare," he said.
"I think we'd be tougher than that."
But shadow home secretary Yvette Cooper said the report showed the Government had "no workable policies" to meet its promise and that its immigration policy was in "complete disarray".
Changes to work and student visas are expected to cut net migration by a respective 11,000 and 56,000 people by 2015, Government figures show.
The Migration Observatory also found that changes to family visas will cut the numbers by 8,000 at most, while plans to make it harder for migrants to settle in the UK are unlikely to reduce migration flows until 2016.
Scott Blinder, senior researcher at the observatory, said: "The Government's current policies only look likely to reduce net migration by about 75,000 at best, which would mean that further reductions of more than 67,000 would be needed to meet the tens of thousands net migration target."
To achieve the 100,000 target, the Government will either have to further tighten immigration rules or "reconsider the target or the time-frame in which they intend to deliver it", the report said.
The Government could also hope that more British and EU nationals emigrate from the UK or that current assessments are "way off the mark". But the report said these two options are "pretty unlikely".
Ms Cooper said: "David Cameron pledged to cut net migration to the tens of thousands just two months ago, saying, 'No ifs, no buts, that's a promise to the British people'. This report shows the Government has no workable policies to meet that promise and David Cameron is not being straight with the British people.
"There is a massive gap between the Prime Minister's rhetoric on immigration and the reality of his policies. It is irresponsible of the Prime Minister to make promises that bear no relationship to the practice."
She went on: "Last week we learnt that their changes to the student visas system would cost the UK economy £2.4 billion and would still only have half the impact on net migration that they promised.
"Recent reports also show the Government is failing to ensure the UK Border Agency has the resources it needs to effectively enforce immigration rules, with 5,000 cuts in staff and no chief executive in place after five months.
"At the heart of their policies is chaos, confusion, rows between ministers and misleading measures which will fail to properly safeguard both the economy and the UK's borders."
Responding to the report, immigration minister Damian Green repeated the Government's pledge to cut net migration to less than 100,000 by 2015.
"For too long the immigration system was allowed to get out of control," he said.
"This Government will tackle abuse of the system and get net migration reduced back down to the tens of thousands in the lifetime of this Parliament."
Earlier, Mr Cameron said the Liberal Democrats were preventing the Tories from taking tougher action on immigration.
"We've all had to make compromises," he told BBC Radio 2's Steve Wright in the Afternoon.
"If I was running a Conservative-only Government I think we would be making further steps on things like immigration control or making sure that our welfare reforms were absolutely making sure that if you're not prepared to work you can't go on welfare," he said.
"I think we'd be tougher than that.
"We make compromises, we make agreements, but as a Government I think we're delivering a lot of good things for the country."
