HSBC, Deutsche Bank and the Bank of Nova Scotia have been accused of rigging the price of silver.
According to J Scott Nicholson, an investor who filed a lawsuit against them in the US, the banks abused their position of controlling the daily silver benchmark by unlawfully manipulating it to reap illegitimate rewards from trading.
This would have been detrimental to other investors in the $5-trillion silver market, who base billions of dollars worth of transactions on the benchmark.
By filing the suit, which claims that the banks violated antitrust laws and the Commodity Exchange Act, Nicholson hopes to represent a class of investors who have bought silver future contracts since 1 January 2007. "The extreme level of secrecy creates an environment that is ripe for manipulation,” he said.
"Defendants have a strong financial incentive to establish positions in both physical silver and silver derivatives prior to the public release of silver fixing results, allowing them to reap large, illegitimate profits."
A spokesperson for the Bank of Nova Scotia told Bloomberg that it would "vigorously defend" itself. HSBC and Deutsche Bank have not commented.
The lawsuit is the latest in a series of metal price-fixing allegations against banks. All three banks have already been taken to court over unlawfully fixing the price of gold, and Barclays Bank was fined £26m by UK regulators earlier this year after one of its traders was found attempting to fix the price of the precious metal.
But previous investigations by the US Commodity Futures Trading Commission have failed to uncover any evidence of such malpractice in the silver market. This included a five-year investigation which was concluded last year.
The benchmark is currently used by everyone involved in trading silver, from mining companies to central banks. But the silver price-fixing system is set to change next month, after London Silver Market Fixing Ltd said in May that it would stop administering the benchmark once Deutsche Bank ends its participation on 14 August.
Earlier this year, Deutsche Bank said that it would no longer take part in the price-fixing markets for gold and silver.