Tusk stated this in Lisbon, during a press conference with Portuguese Prime Minister Antonio Costa.
He warned that the opposite outcome, a so-called Brexit, would threaten to unravel the entire bloc.
“I would like to appeal to the British citizens; I know that for a fact, of almost all Europeans and European leaders; stay with us; we need you.
“Without you, not only Europe but the whole Western and trans-Altlantic community will become weaker.
“There are more signals of dissatisfaction at the EU coming from all of Europe, not only from the UK,’’ Tusk said.
He warned that a British vote in favour of leaving the EU could be the “first step of the whole process of EU disintegration.
He said that the bloc would be foolish to ignore the message being sent by the British referendum.
“I have no doubt that our enemies, internal and external enemies; they will open a bottle of champagne if the result of the referendum is negative for us.
“United we stand, divided we fall.
“The EU and its institutions are “already prepared for the day after the referendum,” Tusk said.
The vote will take place on Thursday, and high-ranking EU officials are due to discuss the outcome on Friday.
All 28 EU leaders are scheduled to meet in a summit early next week. (dpa/NAN)