The artefacts, which date back to the 5th Century BC, are regarded as some of the finest examples of marble sculpture in the world. Thomas Bruce, the 7th Earl of Elgin and British Ambassador to the Ottoman Empire, took them from the Parthenon in Athens between 1801 and 1805.
Greece has long pushed for their return – and now has the support of 18 members of the US House of Representatives, both Democrats and Republicans, who have written to Mr Johnson to outline their concerns.
Their letter states: “The Marbles have been the source of controversy among western allies for many decades.
“Greece has long wanted these Parthenon Marbles back.
“Today we write to you as members of the congressional caucus on Hellenic Issues to urge your government to negotiate with the Greek government in earnest on the return of the Parthenon Marbles to Greece.”
The group urges the return of the sculptures by 2021 – the 200th anniversary of the foundation of the modern Greek nation.
The letter adds: “We remain appreciative of your efforts and good will in support of the historic special relationship between the United Kingdom and the United States, and look forward to strengthening that relationship through the accomplishment of matters such as this.”
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In Greece, the remaining marbles are laid out with gaps for the ones currently in the UK.
Protesters were pictured outside the British Museum, in Holborn, yesterday dressed as statues and carrying signs sharing the hashtag #lostmymarbles.
In June, the Greek newspaper Ta Nea quoted a UK Government spokesman as saying: “The UK’s position on the Parthenon sculptures remains unchanged – they are legally owned by the British Museum.
“This will not be up for discussion in any future trade negotiations.”
Also speaking to Ta Nea last year, British Museum director Hartwig Fischer defended the acquisition of the Marbles, saying: “When you move cultural heritage into a museum, you move it out of context.
“Yet that displacement is also a creative act.”
Mr Johnson himself is unlikely to be swayed by the US politicians if his track record is anything to go by.
In 2015, during a debate with academic Mary Beard at the Methodist Central Hall in Westminster, he said the Marbles had been “rescued, quite rightly, by Elgin”.
A survey undertake on June 4, 2018 with a sample of 2658 British adults indicated broad support for the return of the sculptures.
The YouGov poll suggested 56 percent believed they belonged in Greece, with just 20 percent saying they should stay in the UK, and 20 percent undecided.