Jean-Claude Juncker has insisted he will not seek to block British efforts to claw back powers from Brussels despite David Cameron's failed bid to prevent him becoming European Commission president.
Mr Juncker said any proposals from Westminster would be "taken under consideration", stressing that he wanted the UK to stay in the EU.
The comments could provide some reassurance to Mr Cameron, who had warned fellow European leaders that installing the former Luxembourg premier in the key post risked scotching attempts to reform.
The Prime Minister has pledged a renegotiation of Britain's membership terms followed by an in-out referendum in 2017 if the Tories are in power.
According to a leaked recording reported in the Daily Telegraph, Mr Juncker told MEPs at a meeting in Brussels: "I would like Britain to stay as an active constructive member of the European Union. If Britain puts forward a proposal it will be taken under consideration.
"I am not in principle saying that no kind of repatriation can take place. If Westminster wants to recover competences, OK. If the others agree, it shall be done be done."
Mr Juncker, who is expected to be confirmed as European Commission president during a vote of MEPs next Wednesday, added: "I've never opposed the idea of a well-structured, well-organised, profoundly negotiated repatriation of competences from Brussels to national parliaments.
"I don't want the EU without Britain. Britain is an essential element of policy making in Europe because the British are a common sense and down to earth people."