The Swiss Competition Commission "has today 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," it said in a statement.

The Commission said it had indications that the banks had "possibly concluded illegal competition defying deals" in the trade of precious metals like gold, silver and platinum.

The investigation comes after a raft of banks had been already been punished for manipulating foreign exchange and interest rate benchmarks such as LIBOR. The scandals have fuelled public anger and driven calls for tighter regulation.