Europe's economic and demographic weight in the world is diminishing and its nations will disappear in time unless they stand together in the European Union, European Commission President Jean-Claude Juncker said on Friday.

Speaking at an event to commemorate the EU's landmark Maastricht Treaty, Juncker said that in 10 years Europe's share in the world economy would fall to 15 percent from 25 per cent now. In 20 years, no European countries would be part of the G7 group of the world's top seven economies.

He said while at the start of the last century, Europeans represented around 20 percent of humankind, this had fallen to between 5 and 7 percent at the start of this century and would further shrink to 4 percent by the end of it.

"So those who do think the time has come to deconstruct, to put Europe in pieces, to subdivide us in national divisions, are totally wrong. We won't exist as single nations without the European Union," Juncker said.

He was referring to the rising support for anti-EU rhetoric across Europe, culminating in the British referendum to leave the EU, and the growing popularity of nationalist and populist movements in France, Italy, Poland, Hungary and elsewhere.

Juncker said the migration crisis, which has seen some 1.4 million asylum-seekers arrive in Europe since the start of last year, had destroyed the idea of the EU as a bloc based on commonly agreed rules.

This was because some governments were refusing to comply with quotas put forward by the Commission saying how many refugees they must accept.

"That's something new. For the first time in post-war European history, not all the member states are applying the commonly agreed rules," Juncker said.

"This is against this basic principle that the European Union is a rule-based system. It is no longer," he said.