"If a country like Britain exits, it must be perceived by the outside world as weakening the Union and as a demonstration of the crisis that the EU is undergoing," said Sefcovic on Sunday, Xinhua news agency reported.
He also said that the EU has been considered a perfect form of integration and a model tool to maintain peace on a continent that was troubled by a war every few decades.
Sefcovic said the British referendum played into the hands of populists and Eurosceptics.
"There is a need to explain and defend the benefits and decisions of the Union in a more convincing manner, and also at the national level," stated Sefcovic.
Sefcovic does not believe the outcome of the British referendum can be reversed.
"This would mean the protraction of instability and uncertainty. Maybe we could get to this debate a generation later," he said, adding that there is currently a need to agree with Britain not only on its exit from the EU, but also on further cooperation with it.