World Bulletin / News Desk
Speaking at the Luxemburg University Juncker said the EU could not mediate if only one side in the conflict asked it to do so.
Juncker also said he was "very worried" about other separatist movements which could be encouraged by the Catalan push for independence.
"If Catalonia becomes independent, others will do the same... I would not like the EU to be made up of 100 states within 15 years," he added.
The European Commission earlier voiced for full respect for Spain's constitutional order, ruling out any unilateral declaration of Catalan independence.