He also said the EU would struggle if he became prime minister
European Commission president Jean-Claude Juncker says former London mayor Boris Johnson needs a reality check over claims the EU acts like Hitler.
Mr Juncker said Mr Johnson needs to return to Belgium to understand how the bloc works - and suggested the EU would struggle to work with Mr Johnson if he became British prime minister.
He 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...he would be welcome in Brussels at any time".
Mr Johnson sparked anger earlier this month when he compared the ambitions of the EU for a united Europe to Hitler's attempt to dominate the continent.
Asked about the relationship if Mr Johnson were to become British prime minister, he said: "The atmosphere of our talks would be better if Britain is staying in the European Union".
He was speaking at the start of the G7 summit gathering of world leaders in Japan where British Prime Minister David Cameron is expected to discuss the EU referendum on the sidelines - Brexit it not on the official agenda.
All seven nations, which will also be joined by the presidents of the European Council and the European Commission, are urging British voters to stay in the union.
Vote Leave says it is little more than a global elite attempting to maintain its self-interest, and ignoring the advantages a Brexit could deliver to the British people.
The meeting is being held in Ise-Shima near one of Japan's holiest Shinto shrines amid tight security and a list of pressing international problems.
Leaders of Germany, Italy, the US, Japan, the UK, France and Canada are dividing their two-day meeting into three main sections: economy and trade, foreign policy and climate change.
Discussions will include the prospect of continued sanctions against Russia for its actions in Ukraine, nuclear power, the war in Syria and counter-terrorism.
Concerns about China's continued development of military infrastructure on reclaimed islands in the South China Sea will also be addressed, which is a potential flashpoint for its Asian neighbours.