New rules from regulators, on top of uncertainties over the U.K.’s future relationship with the European Union, are denting the city’s position as the biggest center for gold trading in the world. The changes threaten to push up costs, a key competitive advantage of London’s over-the-counter market.

Even before the regulations come in, the average net daily volume of gold settled by London Precious Metals Clearing Ltd. fell 12 percent in two months to 18.5 million ounces in August. In New York, the British capital’s biggest rival, trading in gold contracts jumped more than 25 percent in the third quarter from the previous three months, with activity during European hours surging 32 percent.