Diplomatic fallout between the UK and Greece over Elgin Marbles?
The sculptures are on display in the British Museum
Rishi Sunak decided to scrap talks with his Greek counterpart after it was felt a reassurance to avoid focusing on the Elgin Marbles was "not adhered to", Downing Street said.
A meeting with Greek prime minister Kyriakos Mitsotakis on Tuesday was unexpectedly cancelled after he spoke publicly about wanting the ancient artefacts, also known as the Parthenon Sculptures, returned to Athens from the British Museum.
The decision has caused a diplomatic fallout, with a source on the Greek side saying Mr Mitsotakis and his team had been left "baffled, surprised and not a little bit annoyed" at the "11th hour" cancellation.
No 10 said it felt Mr Mitsotakis had rowed back on "reassurances" that his administration would not use the UK visit as a "public platform" to lobby for the return of the sculptures.
During an interview on Sunday, Mr Mitsotakis described the current situation as being akin to the Mona Lisa painting being cut in half.
Following the comments, Mr Sunak felt any talks were likely to be "dominated" by the marbles row, according to Downing Street.
Athens has long demanded the return of the historic works, which were removed from Greece by Lord Elgin in the early 19th century when he was the British ambassador to the Ottoman Empire.
The Prime Minister's official spokesman told reporters:
"The UK and Greece relationship is hugely important, from our work together in Nato, to tackling shared challenges like illegal migration and joint efforts to resolve the crisis in the Middle East and the war in Ukraine.
"When requesting a meeting with the Prime Minister this week, the Greek government provided reassurances that they would not use the visit as a public platform to relitigate long settled matters relating to the ownership of the Parthenon Sculptures, which would only serve to distract from those important issues outlined.
"Given those assurances were not adhered to, the Prime Minister felt it would not be productive to hold a meeting dominated by that issue, rather than the important challenges facing Greek and British people.
"The Deputy Prime Minister was available to meet the Greek prime minister to discuss the wider topics and we are disappointed the prime minister opted not to take this meeting."
On Monday, the Prime Minister's official spokesman stressed Mr Sunak's support for the law that prevents the marbles from being permanently returned and suggested he would not be in favour of any loan arrangement.
British Museum chairman George Osborne, a former chancellor, has previously said he is exploring ways for the Elgin Marbles to be displayed in Greece, with speculation that this could involve a loan deal in which part of the set would be sent to Athens.
Mr Sunak's spokesman said the Government "thinks that the museum is the right place for them."
The official has also said that ministers have "no plans" to change the 1963 British Museum Act which prohibits the removal of objects from the institution's collection.
Sir Keir had indicated he would tell the Greek premier that a Labour government would not change the law but that he would not stand in the way of a loan deal that was mutually acceptable to the museum and the Greek government.
Labour criticised Mr Sunak's decision to cancel his meeting with his Greek counterpart.
A Labour Party spokesman said:
"To pick a fight with a Nato ally for the sake of a headline shows just how weak Rishi Sunak is.
"The Prime Minister should have been talking about the economy, immigration, the Middle East, that's what the country would expect from a leader but Rishi Sunak is no leader."