David Cameron's pledge to cut net migration to the "tens of thousands" by the next election is almost certain to be broken, Business Secretary Vince Cable has said.
The Liberal Democrat Cabinet minister said the Tory target was "not sensible" because it was not possible to control European Union migration or Britons leaving or returning to the UK.
The senior Lib Dem, who has a history of speaking out against his coalition colleagues on immigration, said his party had never signed up to the "arbitrary" measure.
In an interview with BBC political editor Nick Robinson for his documentary The Truth About Immigration Cable said: "In our view, certainly the Liberal Democrats' view, it's not sensible to have an arbitrary cap because most of the things under it can't be controlled.
"So it involves British people emigrating - you can't control that. It involves free movement within the European Union - in and out. It involves British people coming back from overseas, who are not immigrants but who are counted in the numbers.
"So setting an arbitrary cap is not helpful, it almost certainly won't achieve the below 100,000 level the Conservatives have set anyway, so let's be practical about it."
Asked if the target was "a bit of a nonsense" Cable said: "I wouldn't use the word nonsense, but the idea it should come down to 100,000 is something the Liberal Democrats have never signed up to because we simply regard it as impractical."
The Prime Minister admitted last month that the crisis in the eurozone has disrupted his plans to get immigration figures down below 100,000 in time for the 2015 general election.
The Prime Minister's election time pledge to limit net migration to the tens of thousands was dealt a severe blow with the release of official statistics showing numbers rose to 182,000 in the year to June, from 167,000 the previous year.
Statisticians blamed a ''significant'' surge in citizens arriving for work from crisis-hit EU countries such as Portugal, Ireland, Italy, Greece and Spain.