Portuguese President Marcelo Rebelo de Sousa delivers a speech for the Portuguese National Day celebration at the Paris City Hall, on June 10, 2016. [Photo/VCG]

LISBON -- Portuguese President Marcelo Rebelo de Sousa said on Tuesday he believes that the United Kingdom would not opt to leave the European Union and that the referendum taking place on Thursday would make the EU "wake up."

"Given my optimism, inherent to my way of being, even if not an excessive optimism, but an optimism based on knowledge of realities, I persist on my conviction that the vote will be favorable to the permanence of the United Kingdom in the European Union," Rebelo de Sousa said at a conference to mark 30 years anniversary since Portugal joined the European Economic Community at the Portuguese parliament.

"The referendum will serve to wake up the European Union from its fearful, inconsequential lethargy, a lethargy that has dominated the European reality in the past years," he added.

Rebelo de Sousa's comments come as United Kingdom voters get ready to head to the polls on Thursday to make the monumental decision of whether they stay in the European Union or not.

He also said he was concerned regarding the possibility of Brexit and pointed out that it would lead to serious consequences not only for the United Kingdom but also for Europe, and for Portugal.

"The United Kingdom is a fundamental piece in this union of nations," he said.