The Greek prime minister will make the Parthenon marbles the key issue in upcoming talks with Boris Johnson in London, arguing the reunification of the “stolen” sculptures is an intergovernmental matter that lies outside the remit of the British Museum. Determined to raise the issue in his first Downing Street visit, Kyriakos Mitsotakis is also expected to emphasise the leaky roof in the London museum’s Duveen Gallery, where the 5th century BC antiquities are displayed, the Guardian has learned. “It’s of great importance to him,” said a well-placed source, refusing to be drawn on whether the treasures, also known as the Elgin marbles, will be the first or last item on Tuesday’s agenda. “Our arguments are very strong. Now is the time to have a dialogue in good faith.” Ongoing maintenance work at the British Museum has, with the coronavirus outbreak, prevented the pieces from being publicly viewed for almost a year. The Acropolis Museum, which was purpose-built to house the classical carvings at the foot of the monument, reopened in May. Sawn off from the Parthenon in 1802 on the order of Lord Elgin, Britain’s then ambassador to the Ottoman Empire, about 80 metres of the monumental frieze is exhibited in London; 50 metres in Athens. Other parts of the 115-block artwork are in museums across Europe. Few Greek leaders have given such priority to the sculptures’ repatriation as Mitsotakis, who has described the UK’s refusal to engage in talks as a “losing battle”. Within weeks of his centre-right government winning office, the politician told the Observer he was prepared to swap priceless artefacts that had never before left Greece in return for the marbles being brought back in time for the nation’s bicentennial independence celebrations this year. Addressing Unesco late on Friday, he said there was “no better time than now” to return the antiquities, long seen as the most significant link, symbolically, between the modern Greeks and their ancestors. “The UK should move to a bona fide dialogue with Greece. And I urge them to do so. After all, this year marks the 200th anniversary of Greece’s war of independence. There could be no better time than now in which to reunite the missing section of the Parthenon sculptures – in their birthplace – in Greece.” The UN agency announced in September that the nation’s decades-long demand for the marbles’ return was of an “intergovernmental character” and not, as successive British governments have claimed, a matter for the British Museum to decide. Campaigners have long complained of London moving the goalposts and hiding behind museum trustees. Tuesday’s talks are the first official face-to-face meeting between the two leaders and the first since Britain’s exit from the EU. Mitsotakis, a Harvard-trained banker whose background is as privileged as Johnson’s, has already signalled he will invoke “what Boris calls Global Britain” when he raises the issue of the marbles. Surveys have repeatedly shown the vast majority of Britons endorsing the return of the sculptures. Athens has been emboldened by changing attitudes to the return of cultural property. In 2019 France responded with unexpected enthusiasm to a Greek request for the repatriation of part of the frieze from the Louvre. But it is very likely the Greek delegation will also appeal to the British prime minister’s sensitivity as a classicist. An admirer of Pericles, Johnson has famously adorned his office with a bust of the soldier-statesman whose building programme, not least the construction of the Acropolis temples, is most associated with the golden age of Athens. Earlier this year, the UK prime minister acknowledged the intensity of feeling on the issue but said repatriation was not in the offing. Speaking to the Greek daily Ta Nea, he said: “The UK government has a firm, longstanding position on the sculptures, which is that they were legally acquired by Lord Elgin under the appropriate laws of the time and have been legally owned by the British Museum’s trustees since their acquisition.” Anglo-Greek relations have been described as excellent, with British diplomats hailing a strategic framework agreement recently signed between two countries as one of the very first post-Brexit deals with an EU member state. But precisely because that is the case, Greek officials say Mitsotakis feels confident that now is the time to roll up his sleeves, put Johnson on the spot and inject new impetus into the west’s longest running cultural row.