The British Museum’s Parthenon Sculptures are not being returned to Athens swiftly enough, Greek Prime Minister Kyriakos Mitsotakis said on Sunday as he prepared to visit British Prime Minister Rishi Sunak this week.

Athens has long advocated for the return of the Elgin Marbles, as they are commonly referred as. Lord Elgin, then envoy to the Ottoman Empire, removed the 75-metre-long Parthenon frieze, 15 metopes, and 17 sculptures in the early nineteenth century.

“We have not made as much progress as I would like in the negotiations,” Mitsotakis told BBC television on Sunday. “I’m a patient man, and we’ve waited for hundreds of years, and I will persist in these discussions.

“We feel that the sculptures belong to Greece and that they were essentially stolen,” Mitsotakis added before playing down the ownership aspect of the discussions and focusing instead on the importance of reuniting the sculptures with those in Athens.

British officials say the works were acquired legally.

Sunak in March ruled out any change to a law that stops the British Museum handing the marbles back to Greece permanently, but the legislation does not prohibit a loan.

George Osborne, a former British finance minister who is chairman of the museum’s trustees, this month expressed hope for a deal that would allow the sculptures “to be seen in Athens”.

Mitsotakis is due to meet Sunak on Tuesday, a day after a meeting Keir Starmer, leader of Britain’s opposition Labour Party, which is riding high in opinion polls ahead of an election expected in 2024.

The Financial Times last week reported that Starmer would not block a “mutually acceptable” loan deal for the sculptures.