Boris Johnson "crossed the boundaries" when he made a "dangerous" comparison between Brussels and Adolf Hitler, the European Council President has said.
Donald Tusk, the former Polish Prime Minister, attacked the former London mayor for "political amnesia" over his controversial suggestion likening the EU to the Nazi dictator's plans for domination of the continent.
It comes after the Chancellor George Osborne endorsed the view of the former military chief Field Marshal Lord Bramall, who described Mr Johnson’s remarks as "simply laughable" and "absurd".
The former Mayor had told the Sunday Telegraph that the EU was an attempt to recreate the Roman Empire’s united Europe. “Napoleon, Hitler, various people tried this out, and it ends tragically,” he told the paper.
“The EU is an attempt to do this by different methods,” he added.
In the extraordinary intervention, during a press conference in Copenhagen, Mr Tusk said he could not "remain silent" in the face of comments by such an "influential" politician.
Mr Tusk added: "When I hear the EU being compared to the plans and projects of Adolf Hitler I cannot remain silent. Such absurd arguments should be completely ignored if they hadn't been formulated by one of the most influential politicians in the ruling party.
"Boris Johnson crossed the boundaries of a rational discourse, demonstrating political amnesia. In some sense, he illustrated a state of mind and emotions of many Europeans, not only from the UK.
When EU compared to plans and projects of Adolf Hitler I cannot remain silent. Demonstrates political amnesia pic.twitter.com/LU4wxNbvd4— Donald Tusk (@eucopresident) May 17, 2016
"In no way, however, can this be an excuse for this dangerous blackout."
On Monday Mr Johnson was also accused of historical inaccuracy by his London mayoral predecessor Ken Livingstone – who is currently suspended from the Labour Party after becoming embroiled in his own Hitler controversy.
“What I said was perfectly true,” Mr Livingstone told the Evening Standard. “But Boris is a lot better informed about Ancient Greece and Rome than about modern history.
“There was never a plan for a United States of Europe under Hitler. What he wanted was actually a Greater Germany that absorbed neighbouring states, with Britain and France rendered subservient.”
But Mr Johnson defended his comments, saying: “Over the last few thousand years there have been all sorts of attempts in Europe to recreate the dream of the Roman Empire and very often that’s been done by force. The EU is different – it’s tried to do it in a more bureaucratic way.
“The problem is there isn’t a single charismatic authority that anyone feels any loyalty to and it’s completely antidemocratic, that’s the problem.
What has the EU ever done for us?
What has the EU ever done for us?
1/7 1. It gives you freedom to live, work and retire anywhere in Europe
As a member of the EU, UK citizens benefit from freedom of movement across the continent. Considered one of the so-called four pillars of the European Union, this freedom allows all EU citizens to live, work and travel in other member states.
2/7 2. It sustains millions of jobs
A report by the Centre for Economics and Business Research, released in October 2015, suggested 3.1 million British jobs were linked to the UK’s exports to the EU.
3/7 3. Your holiday is much easier - and safer
Freedom to travel is one of the most exercised benefits of EU membership, with Britons having made 31 million visits to the EU in 2014 alone. But a lot of the benefits of being an EU citizen are either taken for granted or go unnoticed.
4/7 4. It means you're less likely to get ripped off
Consumer protection is a key benefit of the EU’s single market, and ensures members of the British public receive equal consumer rights when shopping anywhere in Europe.
5/7 5. It offers greater protection from terrorists, paedophiles, people traffickers and cyber-crime
Another example of a lesser-known advantage of EU membership is the benefit of cross-country coordination and cooperation in the fight against crime.
6/7 6. Our businesses depend on it
According to 71% of all members of the Confederation of British Influence (CBI), and 67 per cent of small and medium-sized enterprises (SMEs), the EU has had an overall positive impact on their business.
7/7 7. We have greater influence
Robin Niblett, Director of think-tank Chatham House, stated in a report published last year: “For a mid-sized country like the UK, which will never again be economically dominant either globally or regionally, and whose diplomatic and military resources are declining in relative terms, being a major player in a strong regional institution can offer a critical lever for international influence.
“This discussion is bedevilled by all sorts of artificial media twit-storms or hysteria of one kind or another. There’s a very good argument against the lack of democracy in the EU.
“Over the last 2000 years people have made repeated attempts to unify Europe by force. The EU is a very different project but it is profoundly antidemocratic.”