By Neil MacLucas

ZURICH--Switzerland's competition authority said Monday it has opened an investigation into possible manipulation of foreign currency rates by big banks, including UBS AG (UBS), Credit Suisse Group AG (CS) and Julius Baer Group AG (BAER.VX).

Some of the biggest banks in the world, including Deutsche Bank AG (DB) and Citigroup Inc. (C), have suspended traders since the probe into collusion on market setting of currency rates began last April. Those two banks alone account for just under one-third of all currency trading.

The probe by Switzerland's Competition Commission, known by the German-language acronym Weko, comes after the country's financial markets regulator said in October it was conducting probes of "several Swiss financial institutions" in regard to possible manipulation of exchange rates. Weko opened a preliminary investigation in September last year.

The Weko probe also includes Zuercher Kantonalbank (ZKB.YY), JPMorgan Chase & Co (JPM), Citigroup, Barclays PLC (BC) and Royal Bank of Scotland Group PLC (RBS), the commission said in a statement.

Authorities in the U.S., U.K. and other countries are also examining whether traders have manipulated the foreign exchange market, where more than $5 trillion in currencies change hands each day.

Write to Neil MacLucas at neil.maclucas@wsj.com

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