Cargill Is Behind Big Cocoa Trade That Rattled London Market

The world’s largest agricultural commodities trader is behind a sharp move that upended the London cocoa market.

Cargill Inc. has been building a dominant position in the market since earlier this year, resorting to the ICE Futures Europe exchange to source large amounts of the key chocolate ingredient, according to people familiar with the matter, who asked not to be named because the information is private. Buying more beans through the futures market this month would further cement that position.

The trade comes as two years of shortages dramatically reduced stockpiles, with a measure of reserves relative to consumption falling ...