One of the Elgin Marbles could return to Greece where they were first displayed - but only if the Greek authorities promised to give it back to the British Museum, the institution's director has indicated.
The museum has allowed part of the world-famous marbles to leave London for the first time by lending a sculpture to a Russian Museum, despite fears of a new Cold War between the Kremlin and the west.
Director Neil MacGregor said the museum's trustees would "consider any request from anyone who is prepared to return the object" as one of the sculptures went on loan for the first time.
The statue of the river god Ilissos, found in the Parthenon in Athens nearly 2,500 years ago, has been lent to the State Hermitage Museum in St Petersburg until mid-January.
In a blog on the museum's website, Neil MacGregor said: "The British Museum is a museum of the world, for the world and nothing demonstrates this more than the loan of a Parthenon sculpture to the State Hermitage Museum in St Petersburg to celebrate its 250th anniversary."
The headless marble statue is one of a number of similar items that once decorated the Parthenon temple on the Acropolis and were removed more than two centuries ago by Lord Elgin, a Scottish nobleman, and are displayed in London's British Museum.
But Greece maintains they were taken illegally during the country's Turkish occupation and should be returned for display in Athens - which the British Museum and the Government reject.
It is the first time one of the British Museum's Parthenon sculptures, which represents about a third of the original decoration of the temple, has left the London institution except in wartime.
The sculptures were presented to the trustees by Parliament in 1816 and were a major event in the Museum's early development as a museum of world civilisations.
In October a team of London lawyers, including Amal Clooney, were involved in talks with the Greek government about a potential legal bid for the works.
Her husband, actor George Clooney said it was "probably a good idea" for them to be returned, in response to a question during a press conference to promote The Monuments Men earlier this year.
On BBC Radio 4's Today programme Mr MacGregor said he hoped the Greek government would be "delighted" that the sculpture would be on display to a new audience.
"I hope that they will be very pleased that a huge new public can engage with the great achievements of ancient Greece. People who will never be able to come to Athens or London will now, here in Russia, understand something of those great achievements in Greek civilisation."
Asked if the sculpture could be loaned to a Greek museum, Mr MacGregor said: "The trustees have always been perfectly clear that they are willing to lend anything in the collection, provided it's fit to travel and there's a serious reason, to a place where it could be safe and where it would be returned.
"The Greek government has always refused to borrow, to date, but the trustees' position is very clear that they will consider any request from anyone who is prepared to return the object."
Explaining the loan to the Hermitage at a time of tension between the West and Vladimir Putin, Mr MacGregor said in his blog: "The trustees have always believed that such loans must continue between museums in spite of political disagreements between governments."
He said the sculpture was a "stone ambassador of the Greek golden age and European ideals" and added: "It is a message that Russia, and the whole world, need to hear and I am delighted that the British Museum has been able to lend such a remarkable object."
The reclining statue will go on public display tomorrow in the world-renowned museum, founded in 1764 by Catherine the Great as part of the European Enlightenment.
Sir Richard Lambert, chairman of the trustees of the British Museum, said: "The trustees of the British Museum hold its collection in trust and believe that the great things of the world should be shared and enjoyed by the people of the world.
"The duty of the trustees is to allow citizens in as many countries as possible to share in their common inheritance. The trustees are delighted that this beautiful object will be enjoyed by the people of Russia."
It is the latest example of a long working relationship between heritage and arts bodies in the two countries, which have included British Museum exhibitions on Ice Age art and the Vikings which featured Russian loans.