Johnson made the historical analogy in an interview in the Sunday Telegraph, calling efforts to unify the region tend to end "tragically."
On June 23, voters in the United Kingdom will be asked to decide whether they want the country to stay in the EU.
Johnson made historical comparisons in his interview.
"The truth is that the history of the last couple of thousand years has been broadly repeated attempts by various people or institutions -- in a Freudian way -- to rediscover the lost childhood of Europe, this golden age of peace and prosperity under the Romans, by trying to unify it," Johnson was quoted as saying. "Napoleon, Hitler, various people tried this out, and it ends tragically."
The comments provoked outrage from supporters of the Remain camp.
"Leave campaigners have lost the economic argument and now they are losing their moral compass," Labour Party lawmaker Hilary Benn was quoted by the Guardian as saying. "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 Conservative cabinet minister Iain Duncan Smith told the BBC the article on Johnson was "rather good" and used "some historical parallels" to explain the concept of "greater Europe."
"I think the whole process of trying to drive Europe together by force or by bureaucracy or whatever other means ultimately makes problems, and that's what he was saying."
On national security, 42 percent said it would be better to leave the EU, compared with 38 percent favoring remaining. On the economy, 33 percent said that they would be personally better off with the United Kingdom remaining, compared with 29 percent favoring leaving on that count and 38 percent saying they didn't know.
ComRes interviewed 2,043 British adults online Wednesday and Thursday.
The Bank of England and the International Monetary Fund last week both released analyses on the risks of an exit.