"You may criticize the Parliament, yes, but it is not the Commission which controls the Parliament, it should be the Parliament which controls the Commission", Tajani reminded Juncker.
The exchange at the legislature in Strasbourg, France, was even more awesome since Juncker and Tajani belong to the same EPP Christian Democratic group. "The Parliament is totally ridiculous", he added.
"The fact that there's about 30 members of parliament present in this debate only really illustrates the fact that parliament is not serious", Juncker said.
Juncker took offense at the very low number of MEPs who attended a debate that was meant to review the outcomes of the Maltese presidency which was viewed by many observers as a particular success.
His remarks drew a sharp rebuke from Antonio Tajani, the European Parliament president.
To which Mr Juncker responded: "I will never again attend a meeting of this kind".
Italy's Tajani became president of the parliament in January and served as European Union transport commissioner from 2010 to 2014. We are not ridiculous.
"The Commission is under control of the parliament, but the parliament has to respect even the presidencies of smaller countries, which the parliament is not doing", he said.
But when he took the floor, around 9.15 in the morning, the room was nearly empty. Its 751 members are the only directly elected politicians outside of the individual member states, but in practice a large degree of power is situated in both the Commission and the EU Council.
"If Mr. Muscat was Mrs. Merkel, hard as that is to imagine, or Mr. Macron, a bit easier to imagine, we would have a full house", Juncker said, referring to the leaders of Germany and France, Angela Merkel and Emmanuel Macron.
Tajani: 'Mr President, I will ask you please to change your language.
Manfred Weber, the head of the centre-right EPP group, said, "I am in contact with Antonio Tajani and Jean-Claude Juncker on this subject (of criticisms of the European Parliament)". Juncker, as President of the European Commission, has the duty to report to the Parliament.