The Financial Conduct Authority said in a statement that it had fined Barclays 26.03 million pounds "for failing to adequately manage conflicts of interest between itself and its customers as well as systems and controls failings" in relation to a fixed London pricing of gold over a nine-year period to 2013.
This is a fresh blow for Barclays, which was at the heart of the Libor interest-rate rigging scandal in 2012. The troubled British bank is also facing investigations along with other major lenders over possible manipulation of foreign exchange trades.
The FCA added that it had fined former Barclays trader Daniel James Plunkett 95,600 pounds and banned him from working within any "regulated activity" after noting that on June 28, 2012, he "exploited the weaknesses in Barclays' systems and controls to seek to influence that day's 3:00 pm Gold Fixing and thereby profited at a customer's expense".
The benchmark gold price is set twice daily by four banks, including Barclays, at 10:30 am and 3:00 pm.
"The Gold Fixing is an important price-setting mechanism which provides market users with the opportunity to buy and sell gold at a single quoted price," the FCA said in a statement.
Tracey McDermott, the FCA's director of enforcement and financial crime, added: "A firm's lack of controls and a trader's disregard for a customer's interests have allowed the financial services industry's reputation to be sullied again.
"Plunkett has paid a heavy price for putting his own interests above the integrity of the market and Barclays' customer. Traders who might be tempted to exploit their clients for a quick buck should be in no doubt -- such behaviour will cost you your reputation and your livelihood," she added.