The European Union must be prepared for the real risk that negotiations over Britain's departure from the bloc will end without a deal, the EU's Brexit negotiator said on Monday.

"In the coming months of negotiations the key point is to conclude a withdrawal agreement. We are not yet there and to be clear, not only because of the Irish/Northern Ireland border," Michel Barnier said at a conference in Ireland.

"There is a real risk and we have to be prepared for any options, including no deal. It's not my option."