UK is EU's 'special' case, Barroso says

"I passionately believe that Europe is stronger with the UK as its member, and that the UK is stronger as a member of the European Union than on its own."

But he indicated that the package of exemptions and opt-outs from EU policies secured by the UK could not be made available for other countries.

"We can, and should, find ways to cater to the UK's specificity, inasmuch as this does not threaten the union's overall coherence," Barroso said.

As well as being outside the Schengen passport-free area and holding the right to opt-out of a range of EU justice and home affairs policies, the UK is one of two countries to have an opt-out from the euro.

It has also indicated that, along with a handful of other countries outside the eurozone, it will stay outside the bloc's newly constructed banking union framework to be supervised by the European Central Bank. It is not a member of the EU's bailout fund and was the only country to oppose the Angela Merkel-inspired fiscal compact treaty on debt and deficit levels.

Barroso's remarks may encourage prime minister David Cameron who has promised to renegotiate his country's EU membership terms followed by an 'in/out' referendum if his Conservative party wins next year's general election.

The UK independence party, which calls for the UK to leave the EU, is narrowly ahead of the opposition Labour party, according to polls ahead of the European elections in a fortnight.

Barroso will leave the EU executive in autumn after 10 years as its chief.

The bloc's tortured attempts to ratify the Lisbon treaty following the collapse of the EU constitution dominated his first term in office.

The overhaul of the EU's banking legislation and economic governance rules, and austerity-heavy response to the eurozone debt crisis, were the hallmarks of his second five years.