European Council President Donald Tusk on Tuesday slammed former London Mayor Boris Johnson for comments likening the EU to Nazi leader Adolf Hitler's political project.

"When I hear the EU being compared to the plans and projects of Adolf Hitler, I cannot remain silent," Tusk said during a press briefing after meeting with Danish Prime Minister Lars Lokke Rasmussen.

"Boris Johnson crossed the boundaries of rational discourse, demonstrating political amnesia. In some sense, he illustrated a state of minds and emotions of many Europeans, not only from the UK. In no way, however, can this be an excuse for this dangerous blackout," added the European Council president.

Campaigning for a "Brexit" ahead of the referendum took a scandalous turn on Sunday when Johnson told The Sunday Telegraph that the EU's political project was similar to those of late European dictators, including Hitler and France's Napoleon.

"The EU is an attempt to do this by different methods," he said. "But fundamentally what is lacking is the eternal problem, which is that there is no underlying loyalty to the idea of Europe."

Johnson, who is considered a front-runner to succeed Prime Minister David Cameron as the Conservative Party's leader, has avidly campaigned for the UK to leave the EU, despite the premier's call to remain in the bloc.

On June 23, British citizens will vote in a referendum that will decide the fate of the UK's inclusion in the EU, a move that could have profound economic consequences for the kingdom and the bloc.

'Our common tool'

But Tusk remained optimistic on Tuesday, reaffirming the EU's relevance amid several political challenges facing the bloc.

"Today's atmosphere of uncertainty is fuelled by crises and tensions which have not been provoked by the European Union: the massive influx of migrants, global economic turmoil, terrorism or the aggressive Russian policy," Tusk noted.

"I am convinced that the European Union is our common tool to solve problems," he said.

Fallout

Several leading economists and business leaders have warned of the fallout a "Brexit" would have on the British economy.

Last week, International Monetary Fund Director Christine Lagarde told reporters in London that if the UK decided to leave the EU, the consequences would be "costly in the long run."

"Depending on what hypotheticals you take, it's going to be pretty bad to very, very bad," Lagarde noted.