The European Union’s chief Brexit negotiator warned diplomats on Sunday that “a lot of work remains to be done” in withdrawal talks between London and Brussels.

The European Commission issued a statement shortly after Barnier briefed EU ambassadors on the negotiations, just four days before a key European summit.

“Michel Barnier briefed EU27 ambassadors this afternoon, following constructive technical-level talks with the United Kingdom over the weekend,” it said.

“He will also inform the European Parliament’s Brexit Steering Group this evening. A lot of work remains to be done,” it warned.

Technical talks with British officials will continue on Monday, and on Tuesday Barnier will head to Luxembourg to brief EU ministers preparing the summit.

The European Council summit on Thursday and Friday has been billed as a key date in efforts to negotiate Britain’s orderly withdrawal from the EU on October 31.

If Prime Minister Boris Johnson is not able to come to an agreement with the other 27 EU leaders this month, Britain may crash out without a follow-on treaty.