Britain intends to hold on to the Parthenon Marbles despite Greece's permanent request for the return of the 2,500-year-old sculptures, Boris Johnson signalled.
In an interview with a Greek newspaper published Friday, the UK prime minister insisted the artefacts were legally acquired by Britain.
Since independence in 1832, Greece has repeatedly called for the repatriation of the treasures – popularly known as the Elgin Marbles.
They were stripped from Athens' Parthenon Temple on the Acropolis by Scottish nobleman Thomas Bruce, known as Lord Elgin, in the early 1800s and shipped to Britain.
Elgin sold the marbles to the British government, which in 1817 passed them on to the British Museum where they remain one of its most prized exhibits.
The museum in London has refused to return the sculptures, roughly half of a 160-metre frieze which adorned the 5th century BC monument, saying they were acquired by Elgin under a legal contract with the Ottoman Empire and are part of everyone's "shared heritage".
"The British government has a firm and long-standing position on the sculptures: they were legally acquired by Lord Elgin, in accordance with the laws in force at the time," Mr Johnson told Greek newspaper Ta Nea.
Mr Johnson, who spent Friday in Northern Ireland to see how Britain is helping the nation with its response to the coronavirus pandemic, said the marbles have been legally owned by the British Museum's Trustees since their acquisition.
Mr Johnson's assertion that the sculptures belong to Britain comes as Greece readies to commemorate the bicentennial of the country's 1821 revolution on March 25.
To mark the occasion, Prime Minister Kyriakos Mitsotakis requested the temporary repatriation of the sculptures in exchange for artefacts that he said had never left Greece before.
Mr Johnson, who regularly spends holidays in Greece at his father's house, said that while he understands the strong sentiment of the Greek people and Mitsotakis over the marbles, they would not be returned.
Britain has long argued that the sculptures had been taken by Lord Elgin with permission from the Ottoman Turks who ruled Greece at the time, but Athens insists the marbles were stolen.
Ta Nea said Johnson's comments marked the first time he has spoken on the subject as prime minister.
But his position on the issue was made clear several years ago when, as mayor of London, he hit back at Hollywood star George Clooney for suggesting Britain return the marbles.
Athens, which has received backing from a groundswell of celebrities on the issue, has over the years said it does not wish to pursue legal action to settle the bitter dispute.
It has called for the United Nations cultural agency UNESCO to act as mediator, an offer rejected by the British Museum.
Greece's conservative government has stepped up pressure for a return of the marbles since it took power in 2019, a campaign it said would be intensified by Britain's departure from the European Union.
Greece's culture minister Lina Mendoni last year branded Elgin a "serial thief" who used illegal tactics to take the marbles.