Negotiators worked through the night to reach a deal before a Sunday deadline for Britain to finalise its divorce terms from the EU, ahead of a December 14-15 leaders' summit that will unlock negotiations on a future trade deal.
May's return comes just four days after her last talks with Juncker ended broke up without agreement when her government's Northern Irish allies objected to plans for the future of the border with EU member Ireland.
The British leader arrived at the commission's Berlaymont headquarters shortly before 0600 GMT and was welcomed by Juncker. The pair ignored questions as they posed for a photos before Juncker steered May off with a hand round her shoulder.
In the corridor before the official handshake, Britain's chief Brexit negotiator David Davis greeted Juncker with a hug.
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