Campaigners including former UK Prime Minister Liz Truss and historian David Starkey are among reported signatories to a letter accusing the British Museum of indulging in "covert negotiation" to return the Parthenon Marbles to Greece.
In the letter seen by Sky News, the signatories say the museum is "accelerating" its campaign to hand back the controversial sculptures and call for an immediate halt to negotiations, saying they may seek legal advice on how to stop them.
The marbles once decorated the temple on the Acropolis in Athens, but were removed by Lord Elgin in the early 19th century and acquired by the museum in London in 1816.
Their rightful ownership has been disputed for decades but negotiations have been going on for several years between the Greek authorities and the museum, a process led by George Osborne, who is the chair of the museum, a former Conservative politician and ex-Cabinet colleague of Truss.
Art historian and former National Portrait Gallery chief Nicholas Cullinan was named the new director of the museum last year.