As shareholder voting gets underway on the $100 billion merger of SABMiller (NASDAQOTH: SBMRY) with Anheuser-Busch InBev (NYSE: BUD), the risk of the deal getting blocked by a small group of vocal critics is rising.

Because AB InBev gave special privileges to Miller's two biggest shareholders, tobacco giant Altria and Colombian beverage distributor BevCo, which together own more than 40% of the brewer's shares, a judge ruled they can't vote on the merger. That means the remaining 60% of the shareholders will determine the deal's fate. If the owners of just a quarter of Miller's remaining shares vote no -- a meager 15% of the total outstanding shares -- the deal will die.

While analysts don't think there are enough votes in opposition to do that, the problem is that another major group of investors -- a bloc of hedge funds that owns 20% of the stock -- might not participate either.

According to a report in The Wall Street Journal, hedges funds such as Elliott Management, TCI Fund Management, and Davidson Kempner Capital Management control roughly 20% of Miller's shares, but do so through derivatives and stock options -- vehicles that don't have voting rights. To gain voting privileges, they would have to convert their derivatives and options at a combined cost of about $95 million, an expense they may not want to pay if the deal can be completed otherwise.

If those shares stay on the sidelines, just 40% of Miller's shares would vote, in which case no votes from just 10% of total outstanding shares could kill the deal. Proxy ballots are due by Sept. 26, or votes can be cast at the general meeting that will be held on Sept. 28.

Two hedge funds in particular have actively campaigned against the merger. Aberdeen Asset Management and Vontobel Asset Management, which together own about 2% of Miller's stock, have said AB InBev's offer undervalues the brewer. It's not a charge that Miller's executives necessarily disagree with. The U.K.'s Brexit vote diminished the value of AB InBev's original offer, and though the brewer sweetened the deal a bit to compensate, they say it now qualifies as the very least they consider to be an acceptable valuation. Considering the amount of money Miller expended in preparation for the merger, they voted unanimously in favor of accepting it.

Within the next week, we'll see whether a tiny group of investors is able to thwart one of the largest mergers in history.

A secret billion-dollar stock opportunity
The world's biggest tech company forgot to show you something, but a few Wall Street analysts and the Fool didn't miss a beat: There's a small company that's powering their brand-new gadgets and the coming revolution in technology. And we think its stock price has nearly unlimited room to run for early in-the-know investors! To be one of them, just click here.

Rich Duprey has no position in any stocks mentioned. 

Want to learn how to invest?

Get a free 10 week email series that will teach you how to start investing.

Delivered twice a week, straight to your inbox.