"I am convinced that a rapid agreement on the conditions of the UK's orderly withdrawal, and a transition period, is possible," former French minister Michel Barnier told an Italian parliamentary commission in Rome.
"For that to happen, we would like the United Kingdom to put on the table, as soon as next week, proposals to overcome the barriers."
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Speaking on the eve of a major Brexit speech by British Prime Minister Theresa May, Barnier reiterated that the EU's priorities were securing agreement on Britain's exit bill, the status of continental EU citizens living in Britain and how the border between southern and Northern Ireland will operate.
"We are a few days away from the fourth round of negotiations," Barnier said.
"I am asking myself questions. I'm wondering why - beyond the progress we've made on certain points - there is still today major uncertainty on each of the key issues of the first phase.
"To make progress, we are waiting for clear commitments from the UK on these precise issues.
"We will listen attentively and constructively to Theresa May's important speech tomorrow in Florence."