A long-running dispute between Britain and Greece over ancient treasures has spilled into tension over Brexit after a demand for the return of stolen cultural artefacts was added to the draft of a European Union negotiating mandate.

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

A draft of the 27 EU nations' position on negotiations with Britain on their future relationship, which was seen by Reuters, seeks the "return or restitution of unlawfully removed cultural objects to their countries of origin".

The document did not specify any cultural objects.

However, an EU diplomat said the line was added at the request of Greece, with support from Italy.

Greece's culture minister said last month that Athens would step up its campaign for the return of the Parthenon Marbles from London and expected to win more support from European peers as Brexit diminishes Britain's influence.

However, a British government spokeswoman ruled out the prospect of discussing rehoming the sculptures during next month's trade talks.

A spokeswoman said: "The EU are still finalising their mandate - this is currently in draft.

"The UK's position on the Parthenon sculptures remains unchanged - they are legal responsibility of the British Museum. That is not up for discussion as part of our trade negotiations."

Rhetoric between London and Brussels over the terms of their future relationship has grown sharper this month ahead of negotiations due to start in early March.

The British Museum says the marbles, which are roughly half of a 160-metre frieze that adorned the fifth century BC Parthenon temple, were acquired by Elgin under a legal contract with the Ottoman empire.

Greece says they were stolen.

An EU source, who declined to be named because discussions on the mandate are confidential, said the reference to stolen artefacts was included in a draft of the document as early as last week.

The source said it also had support from Cyprus and Spain and that, Greece's concerns about the marbles aside, EU countries were more broadly concerned about the illegal trade of artefacts through London auction houses.