European Commission President Jean-Claude Juncker | Emmanuel Dunand/AFP via Getty Images Juncker: Europe ‘losing strength’ relative to other regions

European Commission President Jean-Claude Juncker has sounded a warning on the EU's economic decline and said the bloc should face up to the challenge by endorsing "multilateralism" instead of "provincialism."

“We have a strong economy, but it's losing strength. It was 30 percent [of the world's GDP] a few years ago, it is 23 to 25 percent today, and it will fall below the 20 percent level in a few years," he said in a speech at the Catholic University of Louvain-la-Neuve, Belgium, on Thursday evening.

“In 15 years, no EU member country will be any longer a member of the G7" club of the seven major advanced world economies, Juncker said, due to "a growing weakness in Europe."

Referring to the growing opposition to globalization and trade agreements, the Commission chief cautioned against overestimating Europe's ability to master the challenges alone. “We, Europeans, are the inhabitants of the smallest continent in the world, while we still think that we are the greatest of the great,” he said. "That is not true."

Instead, Europe should remain open to economic exchange with the rest of the world and not follow the protectionist course of U.S. President Donald Trump.

"I am, unlike someone who recently had an electoral success, very much in favor of multilateralism and not of withdrawal and provincialism that do not fit the complexity of the today's world," he said.

Juncker also spoke of the U.K.'s decision to leave the EU, calling it "a tragedy.”