COPENHAGEN (AFP) - European Council president Donald Tusk on Tuesday (May 17) criticised the former conservative mayor of London Boris Johnson for having "crossed boundaries" by saying the European Union was behaving like Adolf Hitler.

"I cannot stay silent," Mr Tusk told reporters in Copenhagen. Mr Johnson, a leading campaigner for Britain to leave the EU, had "crossed boundaries" of acceptable political debate, Mr Tusk said.

"Such absurd arguments should be completely ignored if they hadn't been formulated by one the most influential politicians of the ruling party. Boris Johnson crossed the boundaries of a rational discussion, demonstrating political amnesia," Mr Tusk added.

Mr Johnson made his controversial remarks concerning the UK referendum set for June 23 during an interview with the Sunday Telegraph on May 15.

"Napoleon, Hitler, various people tried this out, and it ends tragically. The EU is an attempt to do this by different methods," said Mr Johnson, a populist figure known for his rhetorical flourishes and historical allusions.

Mr Tusk, while promising not to intervene in the British political debate, felt that Johnson's comments were irresponsible.

"The EU may be blamed for many things, but it still remains the most effective firewall against the ever dangerous and often tragic conflicts among the nations of Europe," he said.