Juncker met with May in late April, with reports shortly afterwards branding the meeting a disaster, with the coverage leading May to attack Brussels for interfering the election campaign.
And now, in an interview with German newspaper Handelsblatt, Juncker said he regrets the leaks to press in Europe.
"The fact that this conversation was reported was a serious mistake," he said.
Asked if he had been specifically involved in the leaks, Juncker added: "I am very talented with self-criticism, but I do not want to bother with that".
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The Frankfurter Allgemeine Sonntagszeitung reported details from the late April dinner, citing anonymous sources and claiming that Juncker had left the summit "10 times more skeptical" about the prospects of reaching a deal.
Despite the frosty reaction from Downing Street, Juncker maintained that relationships with the UK are cordial.
"I get along fine with May," he told Handelsblatt. "She's a tough lady".
His words reflected recent comments from the Prime Minister, who vowed to be "bloody difficult" in her talks with Brussels.