HSBC, JP Morgan and Crédit Agricole have been fined a combined €485m (£413m, $519m) by the European Comission for their role in distorting the Euribor benchmark interest rate used to reflect the cost of interbank lending.
FTSE 100-listed HSBC will have to pay €33.6m in fines, while JP Morgan faces a penalty of €337.2m and Crédit Agricole will have to set aside €114.7m to foot the bill of the sanction imposed by the EC after a five-year investigation.
The three lenders were part of a seven-bank cartel which conspired to fix the price of financial benchmarks linked to the euro between September 2005 and May 2008.
"Banks have to respect EU competition rules just like any other company operating in the single market," Margrethe Vestager, the EU's antitrust commissioner, said on Wednesday (7 December), upon outlining the fines handed to the banks.
"The Commission will not hesitate to investigate and sanction any other cartel it may uncover in the future in the financial industry."
The Euribor rate is aimed at providing protection from market volatility in euro-denominated securities, but banks and traders deliberately manipulated the rate for their own personal benefits. Traders used instant messaging services and corporate chat-rooms to exchange confidential information to distort the rate, similarly to what traders found guilty of fixing the London interbank rate (Libor) did.
Because the Euribor rate was calculated from submissions from individual banks, lenders and traders were given the ability to rig it.
The probe into Euribor manipulation suffered a setback in 2013, after the three banks opted against joining a multi-bank settlement with four other lenders including Deutsche Bank and Societe Generale.
The decision, however, meant the trio forfeited any chance of securing a 10% reduction on their fines.
In the same year, EU antitrust regulators issued fines amounting to approximately €1.7bn as part of a multi-bank settlement related to manipulation of the Libor and Euribor benchmarks, barely 18 months after Barclays was fined by British and US authorities for rigging the Libor rate.
Earlier this year, however, the fines were reduced by €218m, as the EU admitted a calculation mistake and slashed the fine it had handed out to Societe Generale.
Following the verdict from the EC, all the three banks have refused allegations they breached anti-trust rules and announced could appeal the decision.
"Crédit Agricole takes note of today's decision of the European Commission in the Euribor case," the French bank said.
"Crédit Agricole firmly believes that it did not infringe competition law. Accordingly, it will appeal the Commission's decision before the European courts.
"Payment of the fine will not affect the 2016 financial statements given the provisions set aside previously."
Meanwhile, HSBC released a statement saying: "The EC's decision relates to allegations of Euribor manipulation and related purported conduct during the course of one month in early 2007," the London-listed lender said.
"We believe we did not participate in an anti-competitive cartel. We are reviewing the European Commission's decision and considering our legal options."
A spokeswoman for JP Morgan added the bank could launch an appeal.
"We have cooperated fully with the European Commission throughout its five year investigation. We did not engage in any wrongdoing with respect to the Euribor benchmark," she said.
We will continue to vigorously defend our position against these allegations, including through possible appeals to the European courts."