Britain would have "zero influence" in global affairs if it left the European Union, the outgoing European Commission President has warned.
Speaking to the BBC's Andrew Marr, Jose Manuel Barroso said: "Do we believe that Britain alone can discuss on an equal footing with the US or with a giant like China?"
Barroso, who will step down as president next month after a decade in power, also said an "arbitrary cap" on migration would "not be in conformity with European rules".
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His comments came after the Sunday Times revealed that Cameron is planning to limit the amount of national insurance numbers issued to low-skilled immigrants to allow him to fulfil his pledge of lowering net migration.
Cameron has also promised to hold an in/out referendum on Britain's membership in the EU by 2017, if the conservatives win the next election.
However, Barroso said that the freedom of movement of people and goods is an essential principle of the European Union and would not be renegotiated.
He questioned what immigration restrictions would mean for the two million British people living in Europe. "Britain has to offer to other European citizens [what] the EU offers to British citizens," he said. "It is a matter of fairness."
However, following his interview, International Development Secretary Justine Greening told Sky News: "Free movement of labour was never really meant to be an unqualified principle, irrespective of how it might have worked on the ground.
"We do need to see action taken in relation to negotiation with the EU," she said.
In a speech due to be delivered later today, Barroso is expected to go even further by saying that Britain's risks isolating itself with its attitude towards Europe, The Guardian reports.
"It would be an historic mistake if on these issues Britain were to continue to alienate its natural allies in central and eastern Europe."
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