PERTH, Australia-- BHP Billiton Ltd.'s shareholders voted overwhelmingly Wednesday in favor of a breakup of the mining giant into two different companies. But its soon-to-be-listed spinoff, known as South32, will start life in a tougher environment than when the plan was first mooted last year.

More than 98% of BHP's shareholders voted at meetings in London and Perth, Australia, to support the split, in which it will separate unwanted assets in commodities such as aluminum, nickel and manganese into South32.

BHP's move to reduce its size is highly unusual in the resources sector, where companies have prided themselves on pursuing growth, often at the expense of profitability. But it comes as a downturn in the price of many commodities has hit mining companies hard, causing their shareholders to press for new strategies that will improve returns.

When BHP announced its intention to split the company in August, some analysts worried it would be jettisoning some of its better-performing assets as prices for the handful of commodities it planned to retain, such as iron ore and coal, weakened.

But analysts have since also downgraded their forecasts for the metals South32 will produce. Concerns over nickel have grown as China's economic growth rate has slowed--the metal is a key component in making stainless steel. A long-awaited recovery in aluminum prices has failed to materialize, while manganese prices have slumped in recent months.

South32 CEO-elect Graham Kerr on Wednesday shrugged off concerns about the new company's value and outlook.

"Commodity prices: It is a cyclical game, they will go up and down," Mr. Kerr said in an interview.

Back in August some analysts estimated South32 might attract a valuation of roughly $15 billion. Its assets had a book value of $12.95 billion on Dec. 31, according to regulatory filings released to the Australian Securities Exchange in March.

But some broker estimates for South32's value have now fallen to as little as $7 billion, analysts at Investec recently said.

"The question about valuation is always interesting," said Mr. Kerr, adding there was a large disparity in analyst estimates given a broad range of views on future commodity and currency values. He declined to disclose the company's own valuation estimate.

A weaker valuation could make South32 vulnerable to an approach from a hungry acquirer. Mining veteran Mick Davis, former chief executive of Xstrata PLC, has raised $5.6 billion for a new mining fund, X2 Resources, which is searching for acquisition opportunities, for example.

Mr. Kerr said speculation around possible potential suitors "is always going to exist."

He said South32 could be a potential acquirer itself, given its low-debt balance sheet, although Mr. Kerr said the company's focus would be on organic growth.

BHP CEO Andrew Mackenzie has argued the separation will allow South32's management to pursue their own tailored strategy, leaving BHP to focus on a core suite of four to five commodities where it benefits from economies of scale and strong market shares.

Mr. Kerr said support from investors for the breakup had "well and truly surpassed" his own expectations.

Shares in South32 will start trading in London and Johannesburg later in May, and in Sydney on June 2. Investors will receive one share for every BHP share they own.

Many U.K.-based shareholders are likely to sell the shares they are gifted in the new company, though, as the company will be ineligible for inclusion in key stock market indexes, Citi analyst Health Jansen said earlier this week.

Anthony Lee, an investor who has held BHP shares for about two years, said he isn't sure if he'll hold on to South32 shares, in part because he's still uncertain about what the company is worth.

"Who knows what the valuation is?" he said at the shareholder meeting in London.

While BHP has vowed its slimmer self will be more efficient and profitable, the breakup will be costly. In March, the miner estimated it would face pretax costs of $738 million to complete the split, including about US$339 million of taxes.

But BHP forecast annual costs savings of around $100 million a year from simplifying its portfolio through the demerger.

Earlier, Mr. Mackenzie told The Wall Street Journal the mining giant remains committed to maintaining or increasing its dividend despite concerns that the policy reduces its financial flexibility at a time of weak metals prices.

Mr. Mackenzie's comments came after Standard & Poor's Ratings Services earlier this week revised its outlook on BHP to negative from stable, saying the miner's payout vow could weaken its financial position given a broader downturn in commodity prices.

"We are remaining committed with confidence to the progressive dividend. At the moment, nothing needs to give," Mr. Mackenzie said.

Scott Patterson in London contributed to this article.

Write to Rhiannon Hoyle at rhiannon.hoyle@wsj.com

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