LONDON--Financier Nathaniel Rothschild, one of the founders of embattled U.K.-listed Bumi PLC (BUMI.LN), said Monday he has resigned with immediate effect from the Indonesian coal miner's board, citing loss of confidence in the chairman and the board.

His resignation comes at a time when Bumi's board is considering a proposal made by the powerful Indonesian Bakrie family that aims to severe the family's ties with the company, following a tumultuous period in which the company's shares have fallen more than 70% since the beginning of the year.

The Bakrie Group, which is run by the powerful Bakrie family, has asked to exit the troubled venture that it had built with Mr. Rothschild by offering to give up its 23.8% stake in Bumi and pay about $1.2 billion for Bumi's mines. The proposed breakup would leave Bumi with no assets other than the cash from the deal and return control of some of Indonesia's most productive mines to the Bakrie Group.

Mr. Rothschild, the 40-year-old scion of a European banking dynasty, said in a resignation letter to Bumi's chairman Samin Tan that the chairman appeared "determined to drive through the Bakries' proposal" before the completion of the independent probe, a move that he said would be a "disgrace" to even contemplate without first concluding the probe.

He said that Mr. Tan was "not trying to protect the interests of minority shareholders. Rather, I believe that you are complicit in their oppression."

Mr. Tan wasn't immediately available for comment.

"These are clearly the personal views of Mr Rothschild," said Bumi's head of corporate affairs in an emailed statement. He said Bumi's independent directors are unanimous in their support that the Bakrie proposal regarding separation should be reviewed although no view has been formed on value. He added that the independent directors have appointed the Rothschild bank to advise them on the proposal put forward. Mr. Nathaniel Rothschild is a member of the family that gave the bank its name.

Mr. Rothschild said he remains determined to fight for the company's investors and believes he can do a better job from outside the company than from within. He said that was a matter of great regret for him that he was party to bringing the Bakrie family to London.

Chris Fong, spokesman and senior vice president for the Bakrie Group said in reaction to Mr. Rothschild's resignation and accusations the Bakrie Group "will pursue to the full extent of the law the allegations that have been presented to us."

He added that the Bakrie group hopes that Mr. Rothschild, which owns 11.9% of Bumi's shares, will return all the shares and other benefits that had been awarded to him as part of the original deal with the Bakrie family.

-Write to Alex MacDonald at alex.macdonald@dowjones.com

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