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 artefacts were legal and recognisable.
They were removed by Elgin between 1799 and 1810 after he received permission from the Ottoman empire, which ruled Greece at the time.
Elgin claimed he was worried about damage being done to the marbles, but their removal was criticised at the time by figures including Lord Byron.
According to a House of Commons briefing paper from 2017, the UK government's position is that 'issues relating to the ownership and management of the Parthenon sculptures are matters for the trustees of the British Museum'.
Last night Mr Johnson's Brexit negotiator David Frost said the UK would rather walk away from talks than accept anything that interfered with 'the fundamentals of what it means to be an independent country'.
The EU is insisting that there can be no 'divergence' from its rules and regulations in areas including financial services and agriculture.
But Mr Frost used a speech Brussels to warn that the hardline position against this already espoused by the Prime Minister was not just posturing.
He made clear the UK wanted a Canada-style trade deal.
But he added that the UK is 'ready for an 'Australia-style' arrangement, with no comprehensive trade deal, if those terms are not granted.
He pointed out that if the UK and EU roles were reversed and the EU was forced to follow Westminster laws 'democratic consent would snap – dramatically and finally'.
Mr Frost told students and academics at the Université libre de Bruxelles: 'We bring to the negotiations not some clever tactical positioning but the fundamentals of what it means to be an independent country.
'It is central to our vision that we must have the ability to set laws that suit us – to claim the right that every other non-EU country in the world has.
'So to think that we might accept EU supervision on so called level playing field issues simply fails to see the point of what we are doing.
'It isn't a simple negotiating position which might move under pressure – it is the point of the whole project.
'That's also why we will not extend the transition beyond the end of this year. At that point we recover our political and economic independence in full – why would we want to postpone it?
'In short, we only want what other independent countries have.'
Frost described how he started his early career in the EU hub Brussels as a euro-enthusiast but soon became disillusioned with the bloc's institutions.
He added that it was perfectly possible to be both an economic competitor and political partner with the EU in the future.
He said the UK was 'not asking for anything special', just a normal trade agreement that the EU has with other countries around the world.
His defiant speech came after French foreign minister Jean-Yves le Drian predicted a bruising battle on a post-Brexit deal.
David Frost said the UK would rather walk away from talks than accept anything that interfered with 'the fundamentals of what it means to be an independent country'
Speaking at the annual Munich Security Conference, he made clear that Brussels will defend its interests when negotiations begin next month.
'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,' he said.
'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 Mr Barnier have both cast doubt on Boris Johnson's aim to reach a comprehensive agreement by the end of the year when the Brexit transition period runs out.
The EU has repeatedly warned Britain cannot expect to enjoy continued 'high-quality' market access if it insists on diverging from EU social and environmental standards.
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.
The bloc's chief negotiator Michel Barnier has said the EU's top priorities are fishing, security and maintaining fair trading conditions for European companies. A fishing boat is pictured above in the English channel