BY DANA CIMILLUCA AND JOHN W. MILLER

LONDON—With Glencore International AG and Xstrata PLC set to unveil their long-awaited tie-up, a big question is whether Xstrata shareholders will support the deal—not a given in light of U.K. investors' recent history of flexing their muscles.

Xstrata, a large U.K.-listed miner of copper, coal and other products, is likely to announce Tuesday it has agreed to merge with Glencore, a Swiss commodities trader that already holds a 34% stake in Xstrata, according to people familiar with the matter. The tie-up would create an industry behemoth with a market value north of $80 billion.

Even though the companies are likely ...