The European Union and Britain are set to begin talks on a post-Brexit free-trade deal next month.

There are many contentious issues — and one European minister has predicted that the two sides will "rip each other apart" before reaching a deal.

The EU will ask UK Prime Minister Boris Johnson to give ancient marble sculptures back to Greece as part of the deal, according to The Times of London.

Greece argues that the Parthenon Marbles, shipped from Athens by the British diplomat Lord Elgin more than 200 years ago, were taken unlawfully and should be returned.

The sculptures, considered one of the great works of ancient civilization, are displayed in a custom-built wing of the British Museum in London.

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The European Union will demand that Britain gives a collection of ancient marble sculptures back to Greece as part of a post-Brexit trade deal.

Greece has long argued that the Parthenon Marbles — also called the Elgin Marbles — were unlawfully removed from the Parthenon temple in Athens in the early 19th century by the British diplomat Lord Elgin.

The marbles — considered among the great works of ancient civilization — are displayed at the British Museum in London in a wing custom-built to accommodate them.

The museum told the newspaper The Times of London that the classical Greek structures were taken from the Parthenon temple lawfully and were "accessible to the 6 million global visitors the museum receives each year."

The Times, however, reported that the EU's draft negotiating guidelines for a trade deal with the UK included a commitment to the "return or restitution of unlawfully removed cultural objects to their country of origin."

Brussels included this clause at Greece's request.

"It is a measure of how Brexit has changed the game that the Greeks feel able to use the trade talks to pursue the Elgin Marbles," an ambassador involved in trade talks with UK Prime Minister Boris Johnson's government told the newspaper.

A Downing Street source told the newspaper The Sun that the marbles were "going nowhere."

"This shows a troubling lack of seriousness about the negotiations on the EU side," the person added.

British and EU negotiators are set to begin talks on a free-trade deal next month. The two sides aim to reach some sort of agreement by the end of the year, when the 11-month Brexit transition period comes to an end.

The question of which country ought to have the Parthenon Marbles is just the latest issue in what are set to be a bruising series of negotiations.

David Frost, the UK's chief Brexit negotiator, said in a speech Monday that the UK would not sign up to EU rules as part of a free-trade deal — and would rather walk away without one.

Fishing is also set to be a thorny subject. Johnson's UK government has pledged to take full control of Britain's fishing waters as part of Brexit. The EU, however, wants European fishing boats to retain access to them.

Jean-Yves Le Drian, France's foreign minister, on Sunday predicted that UK and EU negotiators would "rip each other apart" in talks.

Several senior EU figures including Phil Hogan, the bloc's head of trade, have expressed doubts that a deal can be struck in so short a timetable. EU trade deals usually take at least a few years to be completed.