An ongoing argument between the UK and Greece over ancient artifacts has become inflamed by Brexit tensions after the EU added a demand for the UK to return "unlawfully removed" cultural treasures to its draft mandate for the upcoming withdrawal talks.

The British Museum in London has rejected calls to return the Parthenon Marbles, 2,500-year-old sculptures from Athens that British diplomat Lord Elgin removed in the early 19th century when Greece was under the control of the Ottoman empire.

A draft of the EU's position on negotiations over the UK's future relationship with the bloc calls for the "return or restitution of unlawfully removed cultural objects to their countries of origin."

While the document doesn't specify the Pantheon Marbles, an EU diplomat stated that the demand was requested by Greece, with Italy's support.

Greece's culture minister said last month that the long-running campaign to bring the marbles back to Athens would now be ramped up, adding that they expected to garner more support from their European colleagues as Britain exits the union.

The British Museum claims the Pantheon Marbles were obtained legally by Elgin under a contract with the Ottomans, but Greece says they were taken illegally.

A British government spokesperson said the UK's stance on the marbles, which are around half of a 160-metre frieze that adorned the fifth century BC Parthenon temple, were still "the legal responsibility of the British Museum."

"That is not up for discussion as part of our trade negotiations," she added in a statement.

With negotiations over the EU's future relationship with the UK set for early March, both sides have become more combative in their rhetoric regarding the terms of the withdrawal deal.

An EU source told Reuters that the demand also had backing from Cyprus and Spain, with EU countries raising wider concerns over the illicit trade of artifacts via London auction houses.