Business Secretary Vince Cable, whose Liberal Democrat party shares power with Cameron's Conservatives, said the target was "not sensible" because Britain cannot control migration from the European Union or the number of Britons returning from abroad.
Cable, who has a history of speaking out against his coalition partners on immigration, said his party had never signed up to Cameron's "arbitrary" bid to reduce net migration to under 100,000 before the next general election in May 2015.
"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."
He spoke as a British Social Attitudes survey suggested that 78 per cent of Britons want to see a cut in immigration.
And 56 per cent of those questioned wanted to see a major crackdown on immigration.
Cameron's plans were dealt a heavy blow in November when official figures showed that net migration to Britain rose to 182,000 in the year to June, up from 167,000 the previous year.
Immigration is a hot issue in Britain, where some Conservative backbenchers and right-leaning newspapers have whipped up fears of a massive influx of Romanians and Bulgarians following the relaxation of European Union labour market restrictions on January 1.
The British government rushed through measures in December restricting access to unemployment handouts for new EU migrants, over fears of "benefit tourism".
Despite the media panic, the feared flood of Romanians and Bulgarians had yet to materialise a week into the new year.
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