On Wednesday, the leader of the United Kingdom Independence Party, Nigel Farage, was dismissive of Mr. Cameron’s plan. “It doesn’t sound very tough to me, because under his proposals somebody can come here Jan. 1 from Romania and within 12 weeks be entitled to unemployment benefits,” he told the BBC. “I think that’s outrageous. I would say we are still being far too generous.”

The opposition Labour Party has also tried to look tough on immigration. The shadow home secretary, Yvette Cooper, said on Wednesday that Mr. Cameron was “flailing around” with his proposals and in fact was “playing catch-up” with tough Labour proposals to make the test proving British residency “stronger and clearer.”

Alastair Campbell, former spokesman for Tony Blair, the Labour prime minister, said, “The tone on immigration is ugly.” Mr. Cameron is only feeding support to the United Kingdom Independence Party, not diminishing it, he said. “We have to be really careful not to be driven by a fleeting spasm of public opinion. When all parties feed the idea that it’s going to be big, it makes it seem so.”

Even Mr. Clegg of the Liberal Democrats has warned the Roma already in Britain not to “behave in a way that people find sometimes intimidating, sometimes offensive.” He continued, “We have every right to say if you are in Britain and you are coming to live in Britain and you are bringing up a family here, you have got to be sensitive to the way that life is lived in this country.”

Mr. Clegg was responding to comments from David Blunkett, a legislator and former Labour home secretary who criticized Roma in Sheffield, his hometown, for behaving as if in a “downtrodden village or woodland.” Mr. Blunkett said the numbers of Roma in Britain were underestimated, and added, “We have got to change the behavior and culture of the incoming Roma community, because there’s going to be an explosion otherwise.”

To the distress of the strong anti-Europe faction in the Conservative Party, which has urged Mr. Cameron in vain to flout European Union law, studies have shown that new immigrants to Britain have brought economic benefits and have been net contributors to tax revenues, not a cost to the state. But some immigrants are bound to be taking jobs that might otherwise go to Britons. According to news reports, Home Secretary Theresa May is said to be delaying a report on the costs and benefits of the free movement of people within the European Union due to be published by year-end because so far there is little evidence that immigrants are abusing the benefits system or placing an excessive burden on welfare and public services, including the National Health Service.