By Alex MacDonald

LONDON--The U.K.'s takeover regulator said Wednesday it would restrict the voting rights of key Indonesian shareholders in coal mining company Bumi PLC (BUMI.LN), a move that could affect the outcome of a shareholder battle over how to severe acrimonious ties between Bumi and the powerful Indonesian Bakrie family.

More specifically, the U.K. Takeover Panel wanted to know why it wasn't made aware that key founding shareholders had close ties to each other at the time the deal was announced; it has launched a separate investigation into the matter.

The U.K. Takeover Panel, which is responsible for ensuring fair treatment for shareholders in takeover bids, determined that the influential Bakrie Group was acting "in concert" with Bumi co-founder and Indonesian shareholder Rosan Roeslani. It also said that Bumi Chairman Samin Tan was also a concert party given his joint investment in Bumi with the Bakrie family.

In light of such findings, the panel restricted the Indonesian partners' voting rights to 29.9% of Bumi's outstanding shares even though the three collectively own more than half of the company's shares.

The Bakrie Group made it evident that they were dissatisfied with the voting structure and have lodged a complaint with the panel regarding financier and Bumi co-founder Nathaniel Rothschild's conduct in orchestrating the 2010 reverse takeover that resulted in Bumi's creation.

"As it now stands, we are caught in an unjust situation where holders of some 57% of the equity in Bumi are currently (and collectively) restricted to only 29.9% of the votes," the Bakrie Group said.

By contrast, Mr. Rothschild "currently has some 14.7% of the votes in circumstances where he only subscribed to some 3.16% of ordinary shares," the group added. The conglomerate called on Mr. Rothschild to give up his share bonus that he received upon completing the deal.

Bumi's relationship with the Bakrie family began to fray soon after its 2011 listing when boardroom spats over corporate governance, falling coal prices, and an independent probe into alleged financial irregularities prompted the company's shares to plummet.

The dispute has claimed four board seats so far, including Mr. Roeslani who resigned Wednesday after accepting the U.K. takeover panel ruling that he was a concert party to the Bakrie family given the structure of the original reverse takeover deal.

The Bakrie family offered in October to sever all ties by buying all of Bumi's mining assets for about $1.2 billion plus their shares in Bumi. Mr. Rothschild, a former Bumi board member and large shareholder, submitted an alternative proposal that went beyond the Bakrie proposal to remove all the key current Indonesian shareholders from the company by injecting cash into Bumi.

Julian Horn-Smith, senior independent nonexecutive director, said that after reviewing both proposals carefully, "only part of the Bakrie proposal has support" since the board had no intention of selling Bumi's 85% stake in Indonesian coal mining company PT Berau Coal Energy TBk (BERAU.JK). He also noted that Mr. Rothschild's "proposal is undeliverable," given that Mr. Tan and Mr. Roeslani have said they had no intention to sell their respective stakes to Mr. Rothschild.

Bumi said it continues to work toward separating Bumi from the Bakrie family by selling its 29% stake in Indonesia's largest coal mining company PT Bumi Resources TBk (BUMI.JK) to the Bakrie family.

"We will put this to shareholders as soon as possible in 2013 and outline a clear vision for the Company going forward," Mr. Horn-Smith said.

Meanwhile, Mr. Rothschild said he would continue pursuing discussions with Bumi's three largest Indonesian shareholders, the Bakrie family, Mr. Tan and Mr Roeslani, "in an attempt to reach a consensual solution."

--Eric Bellman, I-Made Sentana and Edhi Pranasidhi contributed to this article

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

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