SYDNEY--A deal worth US$2.68 billion for Woodside Petroleum Ltd. to buy most of Royal Dutch Shell PLC's interest in the company has been cast into doubt after a large number of investors opposed the proposal.

Woodside said Thursday that 71.3% of its shareholders who voted by proxy have approved the deal, below the minimum threshold for approval of 75%.

The Australian oil company said around 59% of its shareholders have voted by proxy, meaning there is still an opportunity for the deal to get over the line at a shareholder meeting in Perth on Friday.

The 41% of shareholders that haven't voted can do so at the meeting, but voting isn't compulsory.

"The general meeting will provide shareholders with the opportunity to vote in person in order to determine a final outcome," Woodside said in a statement.

Both companies agreed in June that Woodside would buy back a 9.5% interest in the company held by Shell, pending shareholder approval. The plan, however, drew scorn from some Woodside investors aggrieved that they wouldn't get to participate in the buyback and access the related tax credits.

The buyback, combined with Shell's sale in June of another 9.5% stake to institutional investors, would see the Anglo-Dutch company cuts its Woodside stake to around 4.5% from 23.1%

Write to Ross Kelly at ross.kelly@wsj.com

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