(Photo: CGTN)

European Union leaders are likely to have to hold an emergency summit in November to consider any Brexit agreement struck with Britain, missing an informal deadline the previous month, diplomats in Brussels said.

Agreeing the terms of Britain's exit from the EU next March, as well as an outline of its future ties with the bloc, is proving a tall order. Delays have stirred talk that Britain could crash out of the EU with no agreements to replace nearly five decades of close cooperation in everything from food standards to space exploration and global diplomacy.

Negotiations on the divorce deal resumed after the summer break on Tuesday, but even the bloc's Brexit negotiator signaled growing expectations that were increasingly unlikely to be wrapped up in time for a regular EU summit in October.

"I'm not going to say (it must come in) October. A few days here or there, beginning of November. But not much later than that, certainly," Michel Barnier told a joint news conference with Britain's Brexit minister, Dominic Raab.

Both men agreed that, while there has been progress in drafting possible cooperation between the EU and Britain on security and defense after Brexit, the Irish border and trade ties were still the key sticking points.

"If we have that ambition, that pragmatism and that energy on both sides, I'm confident we can reach that agreement by October," Raab told the same news conference.

But some Brussels diplomats said the process could even slip into December, leaving little time for ratification of an agreement before Britain becomes the first country ever to leave the EU in March 2019.

The Oct. 18-19 summit of all EU leaders has long been cast as the make-or-break moment for a Brexit deal, leaving enough time for the elaborate ratification process by EU member states and the European Parliament.

But persistent disagreements, mostly over how to avoid border checks between the Irish Republic and the British province of Northern Ireland, have now cast that into doubt.

The sources stressed political infighting between pro- and anti-Brexit factions in Britain as a risk factor, pointing to the annual conference of Prime Minister Theresa May's Conservative Party on Sept. 30-Oct. 3.

Other milestones include a Sept. 19-20 informal gathering of EU leaders in Salzburg, Austria, where May is expected to make her case. The last regular leaders' summit this year is scheduled for Dec. 13-14.

Asked whether it was still the British government's aim to reach a deal in October, a spokeswoman for May said: "That is certainly what we are working towards, yes."