A failure to open the door to countries in that unstable region will slow down reforms in the Balkans, but it could also cause new wars and worsen problems with the trafficking of people and goods, Barroso said.
If the Balkan countries are admitted to the EU, that will be a real test for EU leaders, he added.
EU leaders should reach agreement on the future process of enlargement by the end of 2006 so that the endless debate does not become demoralising for future EU newcomers, Barroso said.
He went on to say that the expansion of the EU to the east would strengthen the bloc globally and enable it to face its main rivals China, Russia, India and the United States.
An enlarged EU is a precondition for a more respected and stronger Europe, he said.
Barroso's comments reflect the growing concern in the EC that several EU members do not want to admit Western Balkan countries in the 25-member bloc, DPA reports.
France, which is supported by the Netherlands and Denmark, has urged the EU to consider its absorption capacity, which refers to financial resources and decision-making mechanisms, before it takes a position on future enlargement.