European Commission President Jean-Claude Juncker has questioned whether Boris Johnson's depiction of the EU is "in line with reality".
Speaking at a press conference at the G7 summit in Japan, Mr Juncker said: "I'm reading in [the] papers that Boris Johnson spent part of his life in Brussels.
"It's time for him to come back to Brussels, in order to check in Brussels if everything he's telling British people is in line with reality. I don't think so, so he would be welcome in Brussels at any time."
Mr Juncker's comments were in response to a question about the former London mayor's comparison of the EU's efforts to unify Europe with earlier attempts by Napoleon and Hitler.
Before his political career, Mr Johnson spent several years in Brussels as EU correspondent for the Daily Telegraph. His articles were known for their euroscepticism.
He told the Telegraph earlier this month: "The history of the last couple of thousand years has been broadly repeated attempts by various people or institutions ... to rediscover the lost childhood of Europe, this golden age of peace and prosperity under the Romans, by trying to unify it.
"Napoleon, Hitler, various people tried this out, and it ends tragically. The EU is an attempt to do this by different methods."