Michel Barnier dismissed as rumors reports Tuesday that both sides had tentatively agreed on a British payment of roughly around 50 billion euros ($59 billion).
"The press has been talking a lot about it these days but there is still work to do, the negotiation is not over," he told a forum organized by the Association of German Chambers of Commerce and Industry.
The British government also insists negotiations over the bill are still underway and no amount has been agreed.
Britain has consistently refused to name a figure for the bill, but Transport Secretary Chris Grayling stressed Wednesday that "we will meet our obligations" to the bloc.
Barnier said the EU's position was not about "punishment" or "revenge," but that the bloc did have to worry about its future budgets with Britain departing.
"We simply want to balance the accounts, as in any divorce," he said.
A mid-December deadline is drawing close for a decision by EU leaders on whether "sufficient progress" has been made in order for the negotiations with Britain to be expanded to include future trade relations. Barnier said there's "still much work to do" but that "we are working day and night with the British negotiators to find an agreement."
Earlier in the day, Barnier told a defense forum in Berlin that before EU leaders meet Dec. 14-15, he hoped to be able to report "that we have reached a sufficient level of progress for the European council to decide to move on to the second stage of negotiating the future relationship."
British Foreign Secretary Boris Johnson said he hoped the offer Prime Minister Theresa May will make at the EU's December summit "will be one that guarantees sufficient progress."
"I think that's what everybody around the table, all the 27 plus us, really want to achieve," he said while attending a meeting in Ivory Coast. "Now is the time to get this show on the road. Let's get the serious talks underway."
Barnier suggested one hurdle he faced was overcoming preconceptions that some in Britain had about what Brexit would, and would not mean, emphasizing that Britain would become a "third nation" in terms of its relationship to EU countries.
"I don't know if British firms have always been told the truth on the consequences of Brexit, but I have a responsibility, as a negotiator, now... to tell the truth," he said. "There are consequences and they must be prepared."
In addition to Britain's financial obligations, Barnier said both sides also need to overcome differences over the future rights of their citizens in each other's nations and the border between the U.K.'s Northern Ireland and EU member Ireland.
Those issues "are the basis for moving ahead, and we need further negotiations," he said.
Sylvie Corbet in Paris and Jill Lawless in London contributed to this story
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