The annual Churchill Symposium was launched in 1996 on the 50th anniversary of Sir Winston Churchill’s speech in Zurich, Switzerland.
During the famous speech, Mr Churchill spoke of European unity and how Europe could rise from the aftermath of World War II.
Mr Barnier gave a keynote speech at this year’s Churchill Symposium on Wednesday.
During his speech, he said that the reality of Britain’s decision to leave the EU was only now being felt.
The UK voted to leave the bloc in 2016 and officially left at the start of this year.
Mr Barnier told the event via video link from Paris: “For many people the real consequences of the referendum are only now starting to sink in.
“The reality, which has become clear for all to see, is that Brexit means recreating trade barriers that had not existed for 47 years.”
Mr Barnier also explained how Brexit should be a lesson for the bloc.
He urged the EU to show its 450 million citizens that they benefit from membership in the union.
The EU’s former Brexit negotiator questioned whether he and his counterpart David Frost understood each other.
He said: “We managed to conclude an agreement (on trade), though I am still not entirely sure we understood each other all the time.”
Mr Barnier put it down to “a certain view on Europe and sharing national sovereignty”.