The Leave campaign has lost its "moral compass" and become "hysterical", two senior Labour politicians have claimed after Boris Johnson compared the European Union with Adolf Hitler's attempts to control the continent.
The former London mayor and Conservative MP claimed the last 2,000 years had seen a number of failed attempts to recreate the "golden age" of the Romans.
In an interview with the Sunday Telegraph, he said: "Napoleon, Hitler, various people tried this out, and it ends tragically. The EU is an attempt to do this by different methods.
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.
"But fundamentally what is lacking is the eternal problem, which is that there is no underlying loyalty to the idea of Europe.
"There is no single authority that anybody respects or understands. That is causing this massive democratic void."
The Shadow Foreign Secretary, Hilary Benn, described the former London Mayor's comments as "offensive and desperate".
"Leave campaigners have lost the economic argument and now they are losing their moral compass," he said.
"After the horror of the Second World War, the EU helped to bring an end to centuries of conflict in Europe and for Boris Johnson to make this comparison is both offensive and desperate."
Former Shadow Home Secretary Yvette Cooper joined the condemnation and called on Mr Johnson to not play "political games with the darkest and most sinister chapter of Europe's history".
"The more he flails around with this kind of hysterical claim, the more he exposes his shameful lack of judgement, his willingness to play the most divisive cynical politics, and the emptiness of his arguments," she said.
"One week it is dog whistle attacks on President Obama. Now he is trying to liken the institution that has kept peace on our continent for decades with Hitler, who pursued the genocide of millions of innocent people.
"All because he is desperately seeking headlines for a desperate campaign."
Former Liberal Democrat leader Lord Campbell said Mr Johnson's comments showed his unsuitability for high office.
"Not content with insulting the President of the United States, he now grossly misrepresents the history, aims and objectives of the European Union which was founded to enshrine democratic values and to prevent a repeat of the fascism which polluted the continent under the Nazis," he said.
But Mr Johnson was supported by the former Conservative former chancellor Lord Lamont who said it was a "fact there were fascist theorists who believed very strongly in a united Europe".
"I don't think (Mr Johnson) was saying people who favour the European Union were comparable to Nazis,” he said.
Another backing Mr Johnson was Ukip leader Nigel Farage, who described him as "a joker" but insisted he was serious enough to become Prime Minister.
"Boris goes on surprising people. They say he can't do this [become Prime Minister], he's a joker – it's like Ronnie Reagan," he told the Mail On Sunday.
"Could he do it? Yes. If you'd asked me six months ago I'd have said no but I've changed my mind."