David Cameron has hit back at criticism by cabinet colleague Vince Cable over his promise to cut immigration to "tens of thousands" of people a year.
The Lib Dem business secretary said the prime minister had been "very unwise" and that such a target was Conservative - not coalition government - policy.
He added that Mr Cameron's comments, made in a speech to Tory activists, "risked inflaming extremism".
But the prime minister denied this and said his words had been "measured".
Labour leader Ed Miliband said ministers had to "get a grip" on immigration and stop fighting amongst themselves.
The Conservatives' 2010 election manifesto calls for "steps to take net migration back to the levels of the 1990s - tens of thousands a year, not hundreds of thousands".
However, the coalition agreement between the Conservatives and Lib Dems pledges only an "annual limit" on people coming to the UK from outside the European Union for economic reasons, making no reference to specific numbers.
But in his speech, which took place in Southampton, Mr Cameron said the government's cap on immigrant numbers would "mean net migration to this country will be in the order of tens of thousands each year, not the hundreds of thousands every year that we have seen over the last decade".
Communities had been affected by incomers unable to speak English and unwilling to integrate, he argued, which had "created a kind of discomfort and disjointedness in some neighbourhoods".
He added: "This has been the experience for many people in our country - and I believe it is untruthful and unfair not to speak about it and address it."
But Mr Cable, who has spoken out on several occasions about the economic dangers of imposing a cap on immigration, criticised the wording of the speech, telling BBC chief political correspondent Laura Kuenssberg: "The reference to the tens of thousands of immigrants rather than hundreds of thousands is not part of the coalition agreement; it is Tory party policy only.
"I do understand there is an election coming but talk of mass immigration risks inflaming the extremism to which he and I are both strongly opposed."
But the prime minister rejected the criticism, saying policy was "sensible and measured" and that, following discussions between the coalition partners on how to reduce immigration without damaging the economy, the issue had been "settled".
"We have a very good and robust policy and this is the policy of the whole government," he said. "This policy is Lib Dem policy. This policy is coalition policy."
'Choice of language'
Mr Cable, however, refused to back down on the issue, when questioned, telling BBC Manchester: "The reference to tens of thousands rather than hundreds of thousands of people is not part of coalition policy."
He added: "My references were about the language used and I stand by that, but it's not a criticism of coalition policy."
Mr Cable said he had meant to make a "positive contribution to the debate".
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image of Laura Kuenssberg Laura Kuenssberg Chief political correspondent, BBC News channel
The strength of Vince Cable's language is really quite unusual, remarkable even.
The coalition's policy on immigration was the result of difficult negotiations between the two parties - the Lib Dems wanted a more relaxed approach to limiting numbers coming here, the Conservatives a tougher one.
Vince Cable personally fought for and secured certain concessions which he felt were needed to prevent a cap damaging British businesses.
After those negotiations, a kind of truce was reached, but Mr Cable clearly thinks David Cameron's comments break that truce.
The business secretary's remarks must also be seen in the context of the upcoming local elections and AV referendum.
This is the first time that two parties who've had to get used to working together have been pitted against each other.
It seems this could be the first proper skirmish.
* Read Laura's thoughts in full
* In full: Cameron migration speech
* Treasury unease on migration cap
Aides to deputy prime minister, and Lib Dem leader, Nick Clegg, who unlike Mr Cable saw a copy of the speech before it was released, said he was "proud" of the work both he and Mr Cable had done on the coalition's "sensible" immigration policy.
But they added: "Cameron's language isn't what we would have used… but he's a Conservative leader talking to Conservative voters in the run-up to an election."
Mr Cable was "entitled to the view" that there had been a truce in the coalition over immigration policy, but that was not Mr Clegg's view, the source said.
Before entering power, the Lib Dems were opposed to a fixed limit on immigration and backed an amnesty for some illegal incomers already living in the UK.
'Get a grip'
Labour says the cap, which was introduced recently, will only cover 20% of non-EU migrants and the government is cutting 5,000 staff at the UK Border Agency.
Mr Miliband said: "It's hard to have a government policy that is clear and coherent if your business secretary, who's in charge of your student visa policy, is saying one thing and actually going out of his way to attack the prime minister.
"The next time he makes a speech why don't they get a grip, have a proper discussion in government, get an agreed policy, because that's the right way to run a government."
UK Independence Party leader Nigel Farage said little would change on immigration because of the UK's "open border" with the EU.
But former Labour minister Frank Field, co-chairman of the cross-party balanced migration group, said Mr Cameron was on the "right course" in requiring all those wishing to settle in the UK to speak basic English and making it "much tougher" for those working in the UK to eventually gain citizenship.
Asked about criticism of the prime minister's approach, he said immigration policy had been out of step with public opinion for many years because it had been determined by a "liberal elite".
"They will be angry that that period is now at an end," he told the BBC.
Sir Andrew Green, Chairman of pressure group Migration Watch, said: "We should remember that Vince Cable speaks for a tiny minority of the public.
"According to our most recent YouGov opinion poll, conducted last November, only 4% of Lib Dem voters agreed that the present level of immigration is best for Britain, 78% wanted 100,000 or less, 19% did not know".