American businesses should be prepared for the EU and British markets to be separate, but the EU will remain a major trading partner with the U.S. despite Great Britain's withdrawal from the EU, the EU's chief Brexit negotiator said Wednesday.

Speaking to the U.S. Chamber of Commerce, chief Brexit negotiator Michel Barnier said 80% of the withdrawal deal between Britain and the European Union has been agreed upon. However, important issues remained unsolved, including the Irish border disputes, he cautioned.

"Brexit is a lose-lose game and there will be costs for both side," said Barnier, although he said the cost will be more substantial for the UK. "This will impact in particular manufacturing and logistics, as well as the agricultural and food sectors."

Meanwhile, he said, the EU will remain a major partner for the U.S. and a "global player," which will not be changed by the UK withdrawal.

His remarks came after days of upheaval in the UK Prime Minister Theresa May secured a hard-won deal on Brexit at a special Cabinet summit last Friday. The deal, which seeks a "free-trade area" with the EU for industrial and agricultural goods, envisions a closer future relationship with the EU than hardcore Brexiteers in May's party want.

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Two prominent cabinet ministers- Foreign Minister Boris Johnson and Brexit Minister David Davis - resigned in succession in protest against the deal.

The political turmoil suggests that "people are waking up to the reality on Brexit," said Dalibor Rohac, a foreign policy research fellow at the conservative American Enterprise Institute. The reality, he said, is that "whatever is the outcome of the negotiations is unlikely to be significantly better in any way for the UK than the status quo."

"An economic cost" is inevitable if the UK wants to be in the European Single Market, which could be giving up the desire to control immigration, Rohac said, citing Norway and Switzerland, both of which participate in the Union's single market without joining as a member states, as examples.

This article was written by Jinman Liu, a reporter at Medill News Service in Washington D.C. The Medill News Service is part of Northwestern University.

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