Rishi Sunak's spokesman confirmed that the Prime Minister had cancelled a meeting with the Greek premier, Kyriakos Mitsotakis, after he allegedly failed to stick to "assurances" that he would not publicly inflame a long-running row about the marbles while on a visit to London.
According to the spokesman, Downing Street was keen to avoid a repeat of a 2021 visit by Greece's then leader, which was overshadowed by the marbles, and had obtained the assurances that Mr Mitsotakis would keep the dispute private.
But on Sunday, the Greek prime minister told the BBC that he would push Mr Sunak for the return of the ancient sculptures from the British Museum, saying the current situation with the rest of the collection in Athens was like cutting the Mona Lisa in half.
The assurance given by Greek officials was “not adhered to” and Mr Sunak felt it would “not be productive” to continue with the meeting, the PM's spokesman told reporters.
Mr Mitsotakis was offered a meeting with Deputy Prime Minister Oliver Dowden instead, but refused.
Greek development minister Adonis Georgiadis said Mr Sunak’s decision to cancel the meeting marked a “bad day” for relations between the countries - two NATO allies who are both battling illegal migration.
Speaking to BBC Radio 4’s World At One programme, Mr Georgiadis said: “I think your Prime Minister, when he will have a second thought, he will understand that the Greek prime minister is an important person.
“The Greek people have re-elected him and put their trust in him. I have to be very honest — what Kyriakos Mitsotakis mentioned in his interview is not just his own opinion.
“It is the single one opinion of 11 million Greek people and I think many more million people around the world.”
But on the wider issue of ownership, Britain insists the marbles were legally obtained by Lord Elgin in the early 19th century when Greece was under Ottoman occupation. And UK law forbids the British Museum’s collection from being broken up.
A loan back to Athens might be possible in exchange for ancient Greek artefacts never before seen in Britain, according to former Chancellor George Osborne, chairman of the British Museum trustees, who have separately been contending with numerous thefts from the sprawling site in Bloomsbury.
However, Mr Sunak's spokesman said that even a loan risked opening the floodgates for the British Museum's collection, much of which was obtained by colonial adventurers in Africa and Asia over the 19th century.
"We do think it is potentially a slippery slope. And that's not something we would support," he said.
“We don't want to see them leave the British Museum, which attracts millions of people from all around the world to come and see them.”
Former culture secretary Lord Vaizey said the loan idea had been gaining traction, “and I don’t think the Prime Minister needed really to intervene in this way”.
“And it hasn’t particularly helped our relationships with Greece,” the Conservative peer added on Sky News. “So it’s a very odd row to have and it didn’t need to happen.”