According to a report by Frankfurter Allegmeine Zeitung (FAZ), Mr Juncker thought the Prime Minister looked “anxious, despondent and disheartened” during the hastily organised dinner last week.
The EU President also thought Mrs May looked “marked” by battles over Brexit with her own Conservative ministers and that she was “a woman who trusts hardly anyone but is also not ready for a clear-out to free herself”, according to the newspaper.
Mrs May is also said to have asked for help to overcome British divisions.
The paper said: “She indicated that back home friend and foe are at her back plotting to bring her down.
According to Faz, Theresa May looked tires, anxious and despondent
“May said she had no room left to manoeuvre. The Europeans have to create it for her.”
“May’s face and appearance spoke volumes, Juncker later told his colleagues. She has deep rings under her eyes. She looks like someone who can’t sleep a wink.
“She smiles for the cameras, it went on, but it looks forced, unlike in the past, when she could shake with laughter.
“Now she needs all her strength not to lose her poise.”
It comes after FAZ reported that Mr Juncker thought Mrs May was “in another galaxy” with her Brexit expectations during a dinner in April at 10 Downing Street.
At the time Mrs May dismissed the dinner leak as “Brussels gossip” but officials on both sides said the report in FAZ did not create an atmosphere of trust which is important to reach a deal.
German Chancellor Angela Merkel was also reported to have been irritated by the leak.
Both sides issued statements after last week’s meeting saying talks were “constructive and friendly” and they agree negotiations should be “accelerated”.
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The EU summit on Thursday and Friday rejected Mrs May’s call for an immediate start to talks on the future relationship.
But leaders made a gesture to speed up the process and voiced hopes of opening a new phase by December.
Some said they understood the Prime Minister’s difficulties in forging consensus in London.
At a keynote speech in Brussels, an exasperated Mrs May, said: “We still have some way to go.”
FAZ claims Juncker thought May looked “marked” over internal fighting within her Cabinet
Despite repeated questions about the Brexit divorce bill, the Prime Minister refused to discuss the amount Britain will have to pay to leave the EU, but did say the UK would only agree to a final settlement when both sides had agreed on their future partnership.
Mrs May and the EU27 leaders spoke about migration, the digital economy and some of the most pressing foreign policy issues including North Korea and Iran at the EU summit last week.
Mrs May, said: “I want a deep and special partnership between the EU and the UK after Brexit. A partnership based on the same set of beliefs.
FAZ also reported that Juncker thought May was “in another galaxy” with her Brexit expectations
“I am ambitious and positive for Britain’s future and for these negotiations.
“But I know we still have some way to go. Both sides have approached these talks with professionalism and a constructive spirit and we should recognise what has been achieved to date.
“While there are a small number of issues about citizens’ rights I’m confident we’re in touching distance of gaining a positive outcome.”