An Egyptian writer has resigned from the British Museum's board of trustees, claiming it is "immovable" on its sponsorship deal with oil company BP.
Ahdaf Soueif also cited the museum's positions on worker relations and the repatriation of cultural artefacts as reasons for her departure.
In a blog post she called on the museum "to take a clear ethical position... on issues of critical concern".
Trustee chair Sir Richard Lambert said the board regretted her decision.
He said Soueif - author of 1999 Booker Prize nominee The Map of Love - had been "a much valued voice" since becoming a trustee in 2012.
BP's sponsorship of the British Museum and other cultural institutions, including the Tate and the RSC, has prompted protests from environmental campaigners.
Last month, Sir Mark Rylance resigned from the Royal Shakespeare Company over its ties with the oil giant.
Soueif said the money BP gives to support British Museum exhibitions such as this year's Troy: Myth and Reality could be attained elsewhere.
She suggested its continued acceptance of such sponsorship was motivated by a desire not to "alienate a section of the business community".
She also claimed this mattered more to the museum "than the legitimate and pressing concerns of young people across the planet".
Sir Richard said BP's sponsorship had "made it possible for [the museum] to put on exhibitions and programming that four million people have seen".
Soueif went on to berate the institution for "rarely speaking" on the vexed issue of repatriation, despite being in "a unique position to lead a conversation".
Sir Richard disagreed with that assessment, saying the museum was "playing a very important part in the debate".
Soueif also criticised the museum for allegedly refusing to engage with outsourced staff whose futures were made uncertain by the 2018 collapse of service provider Carillion.
A museum spokesperson said the collapse had been "a difficult situation" and that it had "made it a priority to re-tender for new service providers".
"Given the timescales involved and the limited resources within this small organisation, bringing the services in-house at very short notice was not a viable option."