Origin Energy Ltd. rejected a backup plan from Brookfield Asset Management Ltd. to acquire the majority of its energy business if investors vote against a full takeover next week.

The revised proposal “is not in the best interests of Origin or its shareholders,” the Sydney-based utility said Thursday in a statement. Brookfield’s offer is “incomplete, complex, highly conditional, and does not provide sufficient certainty.”

Investors are due to vote Monday on a A$19 billion ($12.4 billion) takeover by a Brookfield-led consortium, a deal that’s opposed by Origin’s largest investor. In the event that proposal fails, the Canada-based fund had suggested an alternative under which it would pay A$12.3 billion for Origin’s energy generation and retailing business.