Nick Clegg has launched a furious assault on "Chinese-style" welfare reforms being floated by the Tories.
The Liberal Democrat Deputy Prime Minister condemned the idea of "penalising the young" by limiting benefits to the first two children.
The intervention came after Conservative Work and Pensions Secretary Iain Duncan Smith backed proposals for a fresh crackdown on welfare.
In an interview with the Sunday Times, Mr Duncan Smith said he would like EU immigrants to have to wait for up to two years to claim benefits - rather than the three month period that was introduced on January 1.
He said he had been speaking to other member states such as Germany, Italy and the Netherlands who were supportive of the move.
Sources close to the Cabinet minister stressed that he was merely expressing an aspiration for the future, rather than spelling out a policy.
London Mayor Boris Johnson mooted a similar move last week, but Downing Street insisted it would require treaty change and could not be done immediately.
Mr Duncan Smith hailed the idea of limiting child benefit to the first two children as "brilliant", and said rethinking housing benefit for under-25s could encourage people to take jobs.
Speaking on the BBC's Andrew Marr Show, Mr Clegg insisted he was "no slouch" on reforming the benefits system.
He said he supported efforts to agree further immigrant welfare restrictions with other EU countries, and would "look at" other proposals from his coalition partners.
But he added: "I am not in favour of penalising the young. I am not in favour of a sort of Chinese-style family policy saying that the state says it is okay to have two children, it is not okay to have three children."
Mr Clegg was referring to the one-child policy introduced in China at the end of the 1970s, under which families were harshly fined for every child after their first. The policy has been blamed for increasing forced adoptions, infanticide and abandoned babies.
Mr Clegg told BBC Radio 5 Live's Pienaar's Politics that it was right to insist migrants "jump through hoops" before claiming benefits.
"It is eminently sensible to say that if we can come to an agreement that says you have to jump through certain hoops before you can claim benefits having moved to another European country," he said.
But he warned there was a danger of tit-for-tat changes unless the plans were agreed with other nations.
"The idea that somehow we can apply new criteria to Germans, Fins, Dutch, Austrians you name it, but somehow no new conditions would apply to Brits living in other European Union countries is fanciful," he said.
Shadow work and pensions secretary Rachel Reeves said: "The Government have been playing catch-up after Labour proposed measures to ensure people are coming to UK to contribute, not just to claim benefits.
"We've said that the EU framework needs reform, including to look again at social security and Labour market rules, but that means serious proposals and effective influence in Europe, not empty bluster."
Meanwhile, EU Employment Commissioner Laszlo Andor warned that the UK risks "losing friends" if British politicians continue with the same rhetoric on EU migration.
Mr Andor claimed he was justified to warn about the UK being seen as a "nasty" country for pushing forward measures to restrict migrants' access to benefits.
He told BBC Radio 4's The World At One: "I think I perfectly justified my early warning about unfortunately the way the rhetoric developed in the UK results in losing friends and the risk of a bad image unfortunately is still there."
Mr Andor said he disagreed with Labour's shadow business secretary Chuka Umunna who said freedom of movement should only apply to those with jobs.
He said: "I believe that these rights they apply to EU citizens regardless of having or not having a job at the same time. Exactly at this time when there are such great imbalances in the European economy they should say that the economies, the labour markets of particular countries also provide opportunities for jobseekers."
He called on Britain to do more to deal with youth unemployment and to stop the "disproportionate" blaming of migrants for problems such as crime and homelessness.
He said: "I know that crime in the UK did not begin with the Romanians, homelessness in the UK did not begin with the Bulgarians so it's absolutely disproportionate to connect these local problems with these two countries, the so-called new member states of the European Union, there has to be a focus on resolving these social problems in the UK.
"There should be a more robust UK strategy against youth unemployment, for example, and I think it's very often just misleading if there is a disproportionate attention and focus on the EU migrants from these two countries."
Tory Justice Secretary Chris Grayling said people should not be allowed to move around the EU to claim benefits.
Asked if he agreed with Mr Duncan Smith's proposals, Mr Grayling said: "Yes I do. I have to say that I think that there should be an assumption in the way our migration system works that before you can move from one country to another, before you can start to take back from that country's social security system, you should have made a significant contribution to it.
"I spent two-and-a-half years as employment minister working in Brussels to try and get the European Commission to accept the need for change. Many other member states agreed with the need for change. There is a groundswell of opinion out there that is absolutely behind Iain in what he is saying.
"I think we should push for a clear system that says people should be able to move from one country to another to get a job but actually to move from one country to another to live off the state is simply not acceptable."