EU Brexit negotiator Michel Barnier told ministers from member states on Tuesday that December was the latest that a Brexit deal could be struck if a treaty was to be ready by the time Britain leaves the bloc in March, an EU source said.

Asked to say how late a deal could be struck before a "no deal" scenario would follow, a person familiar with the meeting told Reuters that Barnier calculated backwards from Britain's March 29 exit date and suggested December was the “ultimate deadline.”

British Prime Minister Theresa May will address the other 27 EU national leaders at a summit on Wednesday before they sit down to dinner without her.

EU officials said they expected the leaders to tell May they have little more to offer since talks stalled on Sunday and they will step up preparations for Britain to drop out of the bloc with no deal.

"For now, Britain is negotiating with Britain,” Belgian Foreign Minister Didier Reynders said, referring to May's troubles with her own cabinet and supporters. “We need more weeks to see if we can get a deal. We'll work calmly.”

Germany's Europe minister called on May to “take responsibility and be constructive.” But some diplomats believe May could be unable to move again in negotiations until after she has steered her budget through parliament in early November.

Summit chair Donald Tusk also warned that the risk of a “no deal” dumping Britain out of the bloc and into legal limbo and border chaos on March 29 was greater than ever. He put the onus on May to bring a “creative” solution to break the impasse over the EU-UK land border on the island of Ireland.

Tusk called it a “Gordian Knot,” making clear he did not see May as a contemporary “Alexander the Great,” capable of slicing through the tangle of opposition she faces in London and Belfast over her Brexit plan in order to clinch a treaty with the EU.