A top German politician has warned that UK Prime Minister David Cameron's call for a four-year delay before EU migrants can claim certain UK welfare benefits is "highly problematic".
David McAllister, adviser to Chancellor Angela Merkel on the negotiations with the UK, said some of the UK's other EU reform proposals looked achievable.
But the benefits limit risks violating EU non-discrimination rules, he said.
The UK is to hold an in/out referendum on EU membership by the end of 2017.
Restricting the rights of EU workers in the UK, such as tax credits, is an especially thorny issue. Poland in particular - with a large diaspora working in the UK - has voiced strong opposition.
Freedom of movement for EU workers is a core value of the 28-nation bloc, and national restrictions can be challenged in the European Court of Justice (ECJ).
Mr McAllister said the four-year qualification period for British in-work benefits, such as tax credits, was the trickiest part of the reform package put forward by Mr Cameron.
And "we cannot have quotas for EU workers", he stressed.
But he said there was understanding in Germany regarding UK concerns about abuse of the benefits system by some migrants.
The son of a British civil servant who was based in Berlin, Mr McAllister is an MEP for Mrs Merkel's conservative Christian Democrats (CDU). He was speaking in London, as a guest of the Association of European Journalists.
'Fighting like mad'
Reducing the incentives for EU migrants to move to the UK is seen as a way to cut immigration - something that Mr Cameron has pledged to do.
In the year to March 2015, net migration to the UK reached an all-time high of 330,000, of whom 183,000 were EU citizens.
Speaking in Bulgaria on Friday, Mr Cameron said changing the way the EU operated was "hard, it takes time".
"I expect us to have a full discussion in December but it is going to take longer to get the deal that Britain wants, that Britain needs," he said.
"I want to get the substance right before we hold our referendum and I am fighting like mad to get that done."
EU leaders will meet on 17-18 December, but their discussions are expected to focus on Europe's migrant crisis and terrorism threat.
European Council President Donald Tusk said the summit "should pave the way for a deal in February" for the UK. He said he would be writing to all EU leaders on Monday with his assessment of Britain's proposals.
Besides the restrictions on EU migrants' benefits Mr Cameron's other key demands are:
- Protection of the single market for Britain and other non-euro countries
- Boosting competitiveness by setting a target for the reduction of the "burden" of red tape
- Exempting Britain from "ever-closer union" and bolstering national parliaments