The EU today suggested the Elgin Marbles will be the price of a trade deal - as Michel Barnier dismissed the UK's call for a Canada-style agreement.
The bloc's post-Brexit negotiating mandate is being extended to include a demand to 'address issues relating to the return or restitution of unlawfully removed cultural objects to their countries of origin'.
The ask is believed to have been added at the request of Greece, Italy and Cyprus - and indicates that Brussels intends to play hardball.
Meanwhile, Mr Barnier told reporters in the Belgian capital that the UK's position was 'not like Canada', after Boris Johnson signalled he wants the same terms as the north American country.
Mr Barnier swiped at the 'polemic' coming out of Britain, saying: 'We have proposed a trade agreement with a country that has a very particular and unique close geographical proximity not like Canada, not like South Korea and not like Japan. Very particular.'
The tough stance set the stage for a huge battle when negotiations get under way formally next month.
Greek diplomats told Bloomberg the clause on 'cultural objects' was to do with smuggling of artefacts, but another other official suggested it was a reference to the ancient Greek statues in the British Museum, which were taken from the Parthenon in Athens at the start of the 19th century.
Michel Barnier (pictured with Sadiq Khan in Brussels today) told reporters in the Belgian capital that the UK's position was 'not like Canada'
The Elgin Marbles are a collection of classical Greek marble sculptures, inscriptions and architectural members that were mostly created by Phidias and his assistants
Boris Johnson (pictured) signalled he wants the same trade terms with the EU as Canada
Mr Johnson is already facing the threat that Spain will try to use the negotiations to reopen questions over Gibraltar.
Many Parthenon sculptures have been housed in the British Museum since 1816 after they were bought by the government from Lord Elgin.
Greece has long campaigned for their repatriation, and some UK politicians including Jeremy Corbyn have backed returning them.
But the government insists they were purchased legitimately and have been painstakingly preserved in the UK.
There have been warnings that giving back the artefacts would trigger requests from dozens of other countries for the repatriation of artworks in British museums.
The sculptures used to be on the Parthenon at the top of the Acropolis in Athens.
The Elgin Marbles are a collection of classical Greek marble sculptures, inscriptions and architectural members that were mostly created by Phidias and his assistants.
The 7th Earl of Elgin, Thomas Bruce, removed the Parthenon Marble pieces from the Acropolis in Athens while serving as the British ambassador to the Ottoman Empire from 1799 to 1803.
In 1801, the Earl claimed to have obtained a permit from the Ottoman authorities to remove pieces from the Parthenon.
As the Acropolis was still an Ottoman military fort, Elgin required permission to enter the site.
His agents subsequently removed half of the surviving sculptures, as well as architectural members and sculpture from the Propylaea and Erechtheum.
The excavation and removal was completed in 1812 at a personal cost of around £70,000.
The sculptures were shipped to Britain, but in Greece, the Scots aristocrat was accused of looting and vandalism.
They were bought by the British Government in 1816 and placed in the British Museum. They still stand on view in the purpose-built Duveen Gallery.
Greece has sought their return from the British Museum through the years, to no avail.
The authenticity of Elgin's permit to remove the sculptures from the Parthenon has been widely disputed, especially as the original document has been lost. Many claim it was not legal.
However, others argue that since the Ottomans had controlled Athens since 1460, their claims to the