A front-runner to lead the EU after May’s elections has said the UK cannot challenge the union’s basic principles if it wants to stay a member.
Jean-Claude Juncker said Britain may be able to regain some powers as part of a future process of renegotiation.
But the chances of success depended on what was asked for, he told the BBC.
Prime Minister David Cameron has pledged to negotiate a new deal with the EU and put it to a referendum if the Tories win the next election.
Both Labour and the Liberal Democrats oppose this approach, arguing that a referendum should only be held in the event of further powers being transferred from the UK to Brussels.
Mr Juncker, a former prime minister of Luxembourg, is a leading contender to succeed the current head of the European Commission, Jose Manuel Barroso, after next month’s elections.
The BBC’s Ben Wright said the identity of the next commission chief mattered hugely to David Cameron and his hopes for renegotiation.
Mr Juncker is the choice of the centre-right group in the European Parliament, he added, but Downing Street is known to be sceptical about the veteran federalist, who ruled Luxembourg for 18 years.
Launching his campaign for May’s elections, Mr Juncker said he wanted the UK to remain part of the European Union and accepted it was not the only country wanting powers returned to national parliaments and for the EU to do less.
Mr Cameron has said he wants new rules to end “vast migrations” when new countries join the EU.
But Mr Juncker insisted the basic principle of the free movement of people could not be killed and said Britain could not impose its view.
In a timely boost for the UK, Germany said last month that any future changes to the EU as a whole must be fair to nations not using the single currency.
However, France has said re-writing EU treaties is not a priority for Europe in the foreseeable future.