Parthenon Sculptures, which are originally from the temple of Athena in Greece, were brought to Britain by Lord Elgin
By Katie Razzall, Culture and media editor
Pressure has been growing on the British Museum - and others institutions around the UK - in recent years to address the contested items in their collections.
One of the most high-profile examples is the Parthenon sculptures, the fate of which is the subject of much discussion.
The treasures, also known in the UK as the Elgin Marbles, have been on display in the museum since the 19th Century. The chair of trustees, George Osborne, has been conducting behind-the-scenes discussions with the Greek government.
The British Museum has reiterated to the BBC that talks with Greece about the fate of the sculptures are "ongoing and constructive".
The Parthenon Project, which campaigns for a cultural partnership agreement, advocates the British Museum send the sculptures to the Parthenon Collection in the Acropolis Museum in Athens in return for what it calls "blockbuster artefacts that have never been seen outside Greece before".
This could include the 3,600 year old Mask of Agamemnon and the Kritios Boy.
A spokesperson for the Department for Culture, Media and Sport told the BBC the government has "no plans to change the law" to allow a permanent return.
George Osborne used the phrase "win-win" when speaking to the BBC about the sculptures in February, saying he was looking for a "more co-operative arrangement" after a "quite bitter disagreement" with the Greeks for more than 200 years.
He also defended the institution's retention of the sculptures, calling the British Museum a "museum of our common humanity".
More than four million people visited the British Museum in 2022. The Trustees believe that the Parthenon sculptures are "a significant part" of the story told by the museum "of cultural achievement throughout the world, from the dawn of human history over two million years ago, until the present day".
The classical marble statues were made between 447BC and 432BC to decorate the temple of Athena (the Parthenon) on the Acropolis in Athens.
In the 19th Century, the British Ambassador Lord Elgin was granted permission by the Ottoman Empire, the governing authority in Athens, to remove some of the sculptures.
His collection was transported to the UK and, after a Parliamentary Select Committee in 1816 deemed his actions legal, the Parthenon sculptures were transferred to the Museum by an Act of Parliament.
Today, the British Museum is prevented by the British Museum Act from permanently returning the items to Greece - which is not the only country to contest ownership of items held in museum collections.
One of the most high profile of the contested items has been the issue of the Benin Bronzes, elaborate, highly prized sculptures created by specialist craftspeople in what is now modern day Nigeria.
The Horniman Museum in south-east London signed over ownership of its collection of 72 items forcibly removed from Benin City in 1897, to the Nigerian government in November.
At the time, Nick Merriman, the Horniman Museum director, told me there was a "moral argument" to return them. He said "we're seeing a tipping point around not just restitution and repatriation, but museums acknowledging their colonial history".
Earlier this month, the Rijksmuseum in the Netherlands announced it would be returning six items to Sri Lanka following a decision by the Dutch Secretary of State for Culture and Media.
Acquired during colonial times, the objects include the richly ornamented, ceremonial Cannon of Candy which was looted by troops from the Dutch East India Company in 1765.
A new poll, commissioned by the Parthenon Project, released on Friday, suggests the majority of Brits would back returning the sculptures to Greece in a "cultural partnership".
The YouGov survey suggests 64% would support sending the sculptures to Greece if British museums were lent other unique Greek artefacts in return, while 68% thought UK museums making efforts to agree more of these kind of cultural partnerships would have a positive impact on the relationships between the UK and other countries.
Of the 2,300 respondents to the survey, 69% said if the Parthenon Sculptures returned to Greece and were replaced by other Greek items - this would have no impact on their interest in visiting the British Museum.
Lord Vaizey, Chair of the Parthenon Project Advisory Board and a Conservative peer, called these latest poll findings a "shot in the arm for negotiations, clearly demonstrating that a win-win deal underpinned by a cultural partnership has the backing of the British public".
These issues can be complicated.
The Cambridge Museum of Archaeology and Anthropology holds the second highest number of Benin bronzes in the UK after the British Museum (which has around 900). It had been due to return ownership of 116 items to Nigeria earlier this year but those plans have been delayed.
This is believed to have been in reaction to a statement by the outgoing President of Nigeria who appeared to state that returned bronzes are the property of the Oba, or traditional monarch of the Kingdom of Benin.
It led to some outcry in Germany whose Government signed 1,100 bronzes into Nigerian ownership last July. Critics have suggested they may end up in a palace rather than on full public display in Nigeria. That's been denied.
The University of Cambridge told the BBC it is "in talks with all parties over the implementation of the Council's decision to return Benin Bronzes in its collections".
In terms of the Parthenon sculptures, there is a precedent.
In March the Vatican returned to Greece three carved fragments which originally decorated the Parthenon and had been in the Vatican's collection for more than two centuries.
However, this was not state-to-state restitution. Pope Francis donated the sculptures to Greece's spiritual leader, Archbishop Ieronymos II, the head of the Greek Orthodox Church, who gifted them to the Acropolis Museum.
That museum has been purpose-built within view of the fifth century monument. It displays a portion of the surviving Parthenon frieze, with space left for the British Museum's sculptures.
The Kunsthistorisches Museum in Vienna is also reported to be in talks with the Acropolis Museum about a loan agreement involving two more Parthenon fragments.
So where does this leave the chances of agreement between Greece and Britain?
The Greek prime minister, who has previously met George Osborne to discuss the sculptures, was recently re-elected for a second term. During the campaign, Kyriakos Mitsotakis promised that if he won, he would "pick up again the momentum and build upon the progress that we have made."
The Greek government first made a formal request to the British Museum in 1983 for their permanent return, although less formal demands date back to the 19th Century. It's always been apparent that Greece could never agree to a deal that forced it to accept that the sculptures belong to the UK.
The Parthenon Project proposes that both sides "agree to disagree" about ownership of the sculptures, in order to find a resolution.
The Mayor of London, Sadiq Khan, recently told the Greek newspaper Ta Nea that all those involved should "come to an accommodation to share these wonderful, wonderful Elgin Marbles".
The British Museum told the BBC it believed a "long term partnership would strike the right balance between sharing our greatest objects with audiences around the world, and maintaining the incredible collection we hold at the museum".
The DCMS said: "The British Museum has cared for the Parthenon Sculptures for generations and each year millions of people from across the globe see these treasures free of charge".
It repeated its long-held position, that the museum "is prevented by law from de-accessioning items in its collection, except in very specific and limited circumstances" and that the Government would not be changing that.