Britain has been told it must not "beat up" immigrants from eastern Europe and brand them criminals by the new president of the European Commission.
EU chief Jean-Claude Juncker told a TV debate in Austria there has to be an end to discrimination against Poles, Romanians and Bulgarians.
His comments come in the wake of David Cameron's demands for a welfare squeeze to reduce new arrivals.
Mr Juncker - whose candidacy was bitterly opposed by the Prime Minister - said he would not "shoot down" the reform plans and wanted the UK to remain a member state.
He said: "Especially in Great Britain, which always fought for the enlargement of the European Union, there has to be an end to discrimination against countries just because it goes down well topically when you beat up others.
"I am utterly against behaving as if all Poles, all Romanians, all Bulgarians in the European labour market are of a basic mentality that is criminal. These are people who are working and earning their wages."
Last month, Mr Cameron set out his plans to cut the number of people coming from the EU by banning them from claiming welfare for the first four years after arriving in the UK.
He also said he would deport those who did not find jobs within six months.
Setting out his plans to renegotiate membership if the Tories retain power at next year's general election, he fell short of proposing a formal quota on new arrivals from the EU.
German Chancellor Angela Merkel has already said she is opposed to measures that undermine the free movement of labour within the bloc.
Mr Juncker said: "This fundamental right of free movement of workers cannot be questioned existentially because if you question the free movement of workers, Great Britain has to know that one day the free movement of capital will also be called into question.
"Then it will be the end for London's tax rulings, that will no longer be possible in London.
"For me it is clear that free movement of labour was not enshrined in the treaties so that it could be abused. But it is the national legislatures who should fight against this abuse."
A spokesman for Number 10 said: "The Prime Minister addressed these issues in his speech, stressing the important contribution of immigration to Britain and his support for the principle of free movement.
"But he also made clear that the British people were right to want controlled immigration and that free movement was not an unqualified right."
Shadow foreign secretary Douglas Alexander said: "It's no wonder that David Cameron is getting criticism for his approach to Europe, he's spent the past four years burning bridges with our EU allies when he should have been building alliances.
"The tragedy for Britain is that there is a coalition within the European Union to deliver real reform, but David Cameron simply cannot take this forward because he lacks credibility in Brussels and lacks strength in Westminster."
