Prime Minister Rishi Sunak accused his Greek counterpart of trying to “grandstand” over the so-called Elgin Marbles, during a heated exchange in the UK Parliament that illustrated much about the state of British politics but will do nothing to ease flared tensions between two key allies.

Sunak has faced criticism from across the political spectrum in the UK after he abruptly canceled a meeting this week with Kyriakos Mitsotakis, a move that even the prime minister’s allies have described as petulant and undermining the government’s foreign policy objectives.

Read more: Sunak Escalates Marbles Row, Accuses Greek PM of Bad Faith

It meant that at Prime Minister’s Questions on Wednesday, opposition leader Keir Starmer — whose Labour Party lead Sunak’s Conservatives by about 20 points in polls — faced an open goal as tried to pile on his rival’s discomfort.

“The Greek prime minister came to London to meet him, a fellow NATO member, an economic ally, one of our most important partners in tackling illegal immigration,” Starmer told lawmakers. “But instead of using that meeting to discuss those serious issues, he tried to humiliate him and canceled at the last minute. Why such small politics, prime minister?”

It had the desired effect. A visibly irritated Sunak dug in on the reason his office gave for canceling the Mitsotakis meeting: that the Greek prime minister had reneged on an agreement not to raise the issue of the marbles, known more widely as the Parthenon sculptures, in public during his visit to London.

Greek officials have repeatedly denied any deal existed, while pointing out that Mitsotakis’s comments to the BBC which appear to have rankled Sunak about Greece’s claims to the sculptures merely repeated an established position.

In Athens on Wednesday, Mitsotakis tried to play down the diplomatic spat, saying Greece’s relationship with the UK won’t be affected by Sunak’s decision and describing the cancellation of the meeting as an “unfortunate event.”

His comment came after UK Foreign Secretary David Cameron requested talks with his Greek counterpart Georgios Gerapetritis on the sidelines of the NATO summit in Brussels on Tuesday, in an apparent attempt to de-escalate tensions, according to a person familiar with the matter.

But politics in London, especially in Sunak’s Tory party, are febrile. Senior Conservative figures vehemently oppose the idea that the Parthenon sculptures should be returned to Athens, from where they were taken by British diplomat Lord Elgin in the early 19th century.

Read more: Why Parthenon Marbles Fuel a 200-Year Dispute Between UK, Greece

It helps explain why even as polls show the majority of Britons think the marbles should be returned, Sunak — who is struggling to keep his party together amid a row over immigration — opted to take such a dramatic stance on the meeting, while taking the opportunity to say he has no intention of allowing the sculptures to be removed from the British Museum.

Sunak’s spokesman, Max Blain, told reporters Tuesday the government also sees any loan arrangement for the marbles as “potentially a slippery slope” that would invite demands from other countries for the return of other artifacts.

Still, by raising what was a low-level disagreement between allies into a full-fledged spat, Sunak has again raised questions about his judgment. Critics point to his tetchy answers in interviews and in the House of Commons as evidence he’s ill-suited to a role typically seen as requiring a more diplomatic approach.

At the very least, Sunak has handed Labour another easy attack line, as the focus builds toward a general election expected next year.

“Never mind the British Museum,” Starmer jibed in the Commons, “it’s the prime minister who has obviously lost his marbles.”