INTL FCStone Inc has hiked the amount of cash customers have to deposit with them to trade gold, silver and sterling futures, a relatively rare step that shows financial firms are bracing for volatile trading ahead of Britain's vote on Europe.

In a letter to customers seen by Reuters, the U.S. based mid-sized commodities and forex brokerage said it will charge customers 200 percent of the minimum margin set by CME Group Inc for cleared futures for gold, silver, the British pound and euro currency. It was effective on June 16.

But it is relatively unusual for banks and brokers to charge additional margin for precious metals customers, traders said, illustrating how growing concern about the outcome of the vote and the potential for sharp one-off price moves has extended into commodities markets.