Brussels could challenge the government's attempts to curb migrants' benefits, it has warned.
Employment Commissioner Laszlo Andor warned the Government it "should think twice" about restrictions because it can lead to "conflict".
EU migrants will have to show they are earning at least £149 a week for three months to be eligible for benefits under
changes that come into force from Saturday.
The Hungarian said the EU will look at the details of the reforms before making a decision over whether to launch a challenge.
Cameron could be in hot water with Brussels
Asked if the rules were legal in EU law, he told BBC Radio 4's Today programme: "I believe we will have some difficulties because workers in the European Union have rights.
"This is something you have to understand and you cannot judge the working status of somebody after the level of earning.
"The case law in the European Union actually points to the opposite direction.
"What I understand is that in the last few months some members of the British government did not really mind that what they say is confronting the principles in the legislation of the European Union.
"Government should think twice before launching such new initiatives because, unfortunately, that can lead to a kind of conflict."
Asked if that meant it would be challenged, he replied: "First we need to see in writing what the new system will be and after we can make a conclusion."
Andor's previous criticism of David Cameron's "nasty" looking restrictions on benefits for foreigners led the angry Prime Minister to lodge a formal complaint.
He told the programme: "There is so much in the tabloid newspapers about Romanians and Bulgarians that people probably believe that there is a kind of influx while in reality the Romanians and Bulgarians are just a minority among the so-called EU migrants.
"The EU migrants, those coming from other EU countries to the UK in the last 20 years, never came up to the half of total immigration, so what exactly is the problem and what exactly can be remedied by tinkering with the rules, and the rights and the entitlements of EU citizens should be explained in a more factual way rather than just responding to gut feelings."