Athens has demanded the return of ancient marble carvings taken from Greece in the 1800s

British Prime Minister Rishi Sunak has accused his Greek counterpart of politicizing a longstanding dispute over Greek antiquities held in UK museums, saying the leader only sought to "grandstand" on the issue.

Speaking during a contentious Prime Minister's Questions in the UK Parliament on Wednesday, Sunak explained his decision to cancel a meeting with Greek Prime Minister Kyriakos Mitsotakis earlier this week, claiming he had agreed not to focus on the return of the 5th-century antiquities, known as the Elgin Marbles.

"When it was clear that the purpose of the meeting was not to discuss substantive issues of the future but rather to grandstand and relitigate issues of the past, it was inappropriate," he said.

On Tuesday, Downing Street said Mitsotakis had violated an earlier promise to set aside the decades-old dispute, citing a recent interview with British media in which he urged for the return of the marbles.

Greek Culture Minister Lina Mendoni later slammed London for "barbarism," declaring that "the sculptures are the product of theft," while vowing to reach out to the British Museum, where the carvings are stored.

Labour leader Keir Starmer also criticized Sunak over the diplomatic spat, saying "Never mind the British Museum, it's the prime minister who has obviously lost his marbles." Starmer met with Mitsotakis on Monday, just ahead of the canceled meeting with the prime minister, and later defended Greece as "a fellow NATO member, an economic ally, one of our most important partners in tackling illegal immigration."

The carvings were removed from the Greek Parthenon temple in the early 1800s by British diplomat Lord Elgin - then the ambassador to the Ottoman Empire, which ruled over Greece at the time. They have remained in UK possession since, and Sunak has pledged to never repeal 1963 legislation prohibiting the British Museum from returning the treasures.

READ MORE: Sunak vows to 'take on' critics of Rwanda deportation plan

The Greek government initially demanded the carvings back in 1983, but the UK has repeatedly declined its requests, even rejecting an offer by the UN to mediate the dispute in 2015. Despite opposition from London, however, UNESCO has continued to urge the two countries to resolve the question through "bilateral negotiations on the return and restitution of cultural property."