Jean-Claude Juncker has adopted some barbed diplomatic language in his latest Brexit address, claiming "English is losing importance in Europe".
The outspoken European Commission president made the mocking remark during a conference speech, delivered in French, in the Italian city of Florence in which he accused the UK of "abandoning" the EU.
"I will express myself in French because, slowly but surely, English is losing importance in Europe," he joked to applause at the outset of a speech in which he pledged to hold open but tough negotiations.
"Of course we will negotiate with our British friends in full transparency," he said.
"But there should be no doubt whatsoever that it is not the EU which is abandoning the UK, it is the opposite - they are abandoning the EU.
"And this is a difference which will be felt over the next few years."
The senior Brussels bureaucrat has been the apparent subject of Theresa May's ire after the prime minister accused unnamed "European politicians and officials" of issuing threats aimed at affecting the upcoming UK General Election.
A spokesman for Mr Juncker has said his office was too busy to meddle in the June 8 vote - while European Parliament president Antonio Tajani also rejected the Prime Minister's claim.
Mrs May's pointed attack in an election speech at Number 10 came after a German newspaper published an apparently well-briefed account of her meeting with key Brussels in Downing Street last week.
Mr Juncker was quoted as saying he was "10 times more sceptical" of the likelihood of a successful Brexit after the reportedly fractious dinner in a report that appeared to question Mrs May's knowledge of the scope of the talks.
Mr Juncker's latest public comments on an English decline have been played down by deputy chief EU Commission spokesman Alexander Winterstein at a press briefing.
When asked if Mr Juncker's remark would be constructive, Mr Winterstein said: "You are familiar with the way he speaks and that he does sometimes make light-hearted remarks.
"I don't think one needs to extrapolate from that and reach dramatic conclusions."
Stronger words were meanwhile issued by the Belgian prime minister in the continuing row over Britain's financial commitments in the Brexit process, which this week were reported to have soared to €100bn (£84bn).
Charles Michel said Britain's expectations were "not realistic" and warned Downing Street would be "seriously mistaken" if it thought it could leave the EU without paying a divorce bill.
"Perhaps during the referendum the impression was given that once the Brexit button was pushed everything would take care of itself easily," Mr Michel told the AP news agency.
"Well, that is not true. When you push that Brexit button, there are consequences, there is a bill to pay."