Barclays and HSBC are among seven banks being investigated by Swiss officials amid allegations of price fixing in the precious metals market.
The Swiss Competition Commission (COMCO) said it was looking at whether the banks had concluded "illegal competition agreements" when trading precious metals such as gold, silver, platinum and palladium.
It added: "COMCO has opened an investigation against two Swiss banks, UBS and Julius Baer, as well as against the foreign financial institutions Deutsche Bank, HSBC, Barclays, Morgan Stanley and Mitsui."
The investigation is expected to be completed by 2017.
Both Barclays and HSBC have already been fined in recent months for unrelated offences.
Barclays was hit with a massive £1.5 billion fine in May for rigging foreign exchange rates, while HSBC was ordered to pay £28 million in June over failures relating to alleged money laundering at its private Swiss bank.
Share prices in both banks were down slightly in early trading.