It appeared to dash British hopes that a summit later in the week could launch negotiations on future trade ties.
May's office said the meeting was long planned but gave no details. The dinner was not on Juncker's agenda published on Friday, however, and EU officials said it had also not been on the schedule for Brexit negotiator Michel Barnier, who will be at the dinner along with his British counterpart, David Davis.
After talks with Davis last week, Barnier said negotiations were deadlocked, notably over London's refusal to detail what it was offering to pay Brussels after May tried to revive the talks last month by promising Britain would honour its EU commitments.
As a result, Barnier told EU leaders not to launch the talks on a future relationship that May has demanded. As time ticks down to withdrawal in March 2019, concern is rising across Europe that the process may collapse.
In response to suggestions from Barnier, EU governments have agreed, however, that the summit on Thursday and Friday should give him a green light to explore a possible post-Brexit transition period - although only in internal discussions within the EU, not with the British negotiators themselves.
Even that has run into resistance, notably from heavyweights Germany and France. They insist further progress in the British divorce package is required for any such gesture to be made to May, who is struggling to unite her own government behind her plan to reach a deal on a two-year transition during which Britain could stay in the single market and customs union.
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