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4 year oldFrench Foreign Minister Jean-Yves Le Drian predicted the two sides would "rip each other apart" as they strove for advantage in the negotiations.
He also said it would be tough for the UK to achieve its aim of agreeing a free trade deal by the end of the year.
The UK government said it wanted a deal based on "friendly co-operation between sovereign equals".
Boris Johnson's chief Brexit negotiator is expected to give more details of the UK's negotiating aims in a speech in Brussels later.
David Frost is expected to say the UK will be happy with a trade deal based on that agreed by the EU with Canada in 2016 but to rule out any form of regulatory alignment with the bloc from 2021 onwards.
The UK formally left the EU two weeks ago but still trades like a member under a transition period which ends on 31 December.
Talks on their future relationship are set to begin next month once the EU's 27 members have agreed the bloc's negotiating mandate.
Speaking at a security conference in Munich on Sunday, Mr Le Drian said the two sides were far apart on a range of issues.
He said: "I think that on trade issues and the mechanism for future relations, which we are going to start on, we are going to rip each other apart.
"But that is part of negotiations, everyone will defend their own interests."
Mr Le Drian, a close ally of President Emmanuel Macron, is the latest senior EU figure to warn that the negotiations will be difficult.
European Commission President Ursula von der Leyen and chief negotiator Michel Barnier have both cast doubt on Boris Johnson's aim to reach a comprehensive agreement by the end of the year.
The EU has repeatedly warned that the UK cannot expect to enjoy continued "high-quality" market access if it insists on diverging from EU social and environmental standards.
Last week the European Parliament called for the UK to follow EU rules in a host of areas, such as chemicals regulation, food labelling and subsidies for companies, as part of a process of "dynamic alignment".
But UK ministers have repeatedly ruled out such a close regulatory relationship.
There is expected to be a particularly tough fight over fishing rights, with the EU insisting continued access to UK waters must form part of any agreement.
Mr Johnson, in turn, has said the UK will act as an "independent coastal state" taking control of its own fisheries.
A Downing Street spokesperson said: "Our approach is clear - we are not asking for anything special, bespoke or unique, but are looking for a deal like those the EU has struck previously with other friendly countries like Canada.
"We want a relationship based on friendly cooperation between sovereign equals, one centred on free trade and inspired by our shared history and values."
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