Donald Trump has made critical remarks against the President of the European Commission.

Speaking to students at a conference in Luxembourg on Friday, Jean-Claude Juncker said the 70-year-old former reality TV star was ignorant of the EU and its workings, and would "waste time for two years" while touring a world "he is completely unaware of".

"The election of Trump poses the risk of upsetting intercontinental relations in their foundation and in their structure," said Juncker, one of Europe"s most powerful political figures.

Juncker also warned against the "pernicious" consequences of Trump"s statements on security policy.

He also recalled a Trump statement in which he seemed to think that Belgium, the country that hosts the headquarters of the EU and NATO, was a city.