Royal Bank of Scotland and Barclays are among several European and US banks to be hit with fines totalling 99 million Swiss francs (£78m, $96.4m) for attempting to rig Libor interest rates.
The fines were handed down after a four-year probe by Switzerland's competition commission, Comco, which found a number of banks "operated a cartel" to rig euro and yen derivative interest rates.
This $450trn market is used to set prices from everything from residential mortgages to large corporate transactions.
Along with the British lenders, US giants JP Morgan Chase & Co and Citigroup, German group Deutsche Bank, French player Societe Generale and Swiss bank Credit Suisse were also handed down penalties. The offences occurred between 2005 and 2010.
"The cartel aimed at distorting the normal course of pricing components for interest rate derivatives in euro," said Comco.
"Traders of different banks occasionally discussed their bank's submissions for the calculation of the Euribor as well as their trading and pricing strategies."
RBS said: "We are pleased to have settled this legacy issue. The culture at RBS has changed dramatically in recent years. We are determined to put these issues behind us and build a bank that is fully focused on the best interests of our customers."
However, the Swiss regulator said HSBC, Lloyds and City of London brokers Icap, Tullett Prebon and RP Martin all still remain under its investigation.
These fines mark the latest in a long line of bank penalties for rate-rigging scandals that have come to light since the 2008 global financial crisis.
Earlier this month the European Commission announced fines totalling €485m (£409 million) for HSBC, JPMorgan and Credit Agricole for the part they played in conspiring to rig interest rate derivatives.
The rate-rigging scandal has seen banks hit with hundreds of millions of pounds in fines from a variety of regulators across the world, while medium-level traders from Barclays and UBS have been jailed.