European Council president Donald Tusk has warned that a UK vote to leave the EU could threaten "Western political civilisation".
Mr Tusk said a vote to leave the EU would boost anti-European forces.
"As a historian I fear Brexit could be the beginning of the destruction of not only the EU but also Western political civilisation in its entirety," he told the German newspaper Bild.
UKIP MP Douglas Carswell said the Remain campaign was "falling apart".
The UK votes on whether to remain in the EU or leave on 23 June.
Mr Tusk said everyone in the EU would lose out economically if Britain left.
"Every family knows that a divorce is traumatic for everyone," he said. "Everyone in the EU, but especially the Brits themselves, would lose out economically."
In the interview he also said Turkey would not become a member of the European Union "in its current state".
Leave campaigners have regularly accused Remain of scaremongering after repeated warnings from high-profile figures against leaving the EU.
