Britain is the legitimate owner of the Elgin Marbles, Prime Minister Boris Johnson insists.
Greece has called for the repatriation of the frieze – taken from the Parthenon temple in Athens – for more than 180 years.
But the British Museum has always refused the request.
It argues the marbles were bought though a legal contract with the Ottoman Empire.
Mr Johnson, an Oxford University Classics’ scholar, said he understood the feelings of many Greeks about the issue.
However, Britain had a “firm and long-standing” position about the sculptures he told the Ta Nea newspaper.
He added: “They were acquired legally by Lord Elgin, in line with the laws that were in force at that time.”
