THE outgoing president of the European Commission will today warn David Cameron he could make a "historic mistake" by alienating eastern European countries as he attempts to renegotiate the UK's links with Brussels.

With the prospect of a referendum on leaving the European Union a possibility if the Conservatives keep a grip on power after the 2015 General Election, Jose Manuel Barroso will say individual countries like the UK could not retain "even marginal relevance" in a globalised world without the combined weight of the other member states.

The Prime Minister has promised to reform the UK's relationship with the EU ahead of an in/out referendum by the end of 2017 if he remains in office.

Mr Barroso will tell an audience at foreign affairs think-tank Chatham House that the European Commission will "constructively" engage in the discussions - but Mr Cameron will require allies in other EU governments to succeed.

Mr Barroso will warn: "It is an illusion to believe that space for dialogue can be created if the tone and substance of the arguments you put forward question the very principle at stake and offend fellow member states.

"It would be an historic mistake if Britain were to continue to alienate its natural allies in central and eastern Europe, when you were one of the strongest advocates for their accession."