European Council president Donald Tusk has warned that a UK vote to leave the EU could threaten "Western political civilisation".
"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".
He tweeted: "Why hasn't Western civilisation come to an end already seeing as how most countries are self governing?"
Read more: Donald Tusk: Brexit could destroy Western political civilisation - BBC News