Reports in the German press gave details about the Downing Street dinner after which Mr Juncker accused Mrs May of “living in another galaxy” and said he left the talks “10 times more sceptical than I was before” that a deal could be struck.
But Mr Juncker’s conduct has not gone down well in Berlin where Mrs Merkel was hoping for a less confrontational start to Brexit negotiations with the UK.
The German Chancellor believes Mr Juncker or his chief of staff Martin Selmayr briefed journalists about the private conversation with Mrs May and accused them of ”fuelling the flames” before Brexit talks have even got under way.
According to reports in Der Spiegel she said: “It was not particularly helpful to fuel the flames like this.”
The Frankfurter Allgemeine Sonntagszeitung said Mr Juncker repeated his claims that Mrs May was “from another galaxy” and his “having delusions” during a telephone call with Mrs Merkel after the confrontational Downing Street dinner date.
The Prime Minister was outraged by Juncker’s statements and said she was prepared to walk away from the Brexit negotiating table with no deal at all rather than a bad one.
She told the BBC she would be a “bloody difficult woman” when dealing with Mr Juncker in future talks.
She said: “During the Conservative Party leadership campaign I was described by one of my colleagues as a bloody difficult woman.
“And I said at the time the next person to find that out will be Jean-Claude Juncker.”
Mrs May has made it clear she does not want leaks about the Brexit negotiations but many doubt that such secrecy is achievable when Brexit issues affect the interests of 27 other EU countries.
Among the many potential sticking points are the size of the UK’s Brexit bill.
Brexit Secretary David Davis accused the European Commission of “trying to bully the British people” with an £85bn divorce bill from the EU.
Mr Juncker said: “We will negotiate fairly with our British friends, but let’s not forget that it is not the EU that is abandoning the UK – it is the UK that’s abandoning the EU, and that makes a difference.”