© Reuters / Yuriko Nakao
Some of the world's biggest banks - UBS, Citigroup, JPMorgan Chase, Royal Bank of Scotland, and HSBC have agreed to pay out $3.4 billion to settle an investigation into the institutions' rigging of foreign exchange rates.

The probe accused banks of tampering with currency interbank rates on the largely unregulated $5.3 trillion-a-day foreign exchange market.

"The setting of a benchmark rate is not simply another opportunity for banks to earn a profit. Countless individuals and companies around the world rely on these rates to settle financial contracts, and this reliance is premised on faith in the fundamental integrity of these benchmarks," Aitan Goelman, Director of Enforcement at CFTC, said in a press release on Wednesday.

"The market only works if people have confidence that the process of setting these benchmarks is fair, not corrupted by manipulation by some of the biggest banks in the world," Goelman added.

US, UK, and Swiss regulators ordered the fines in tandem. The UK's Financial Conduct Authority (FCA) slapped the six banks with $1.7 billion in fines and the US Commodity Futures Trading Commission (CFTC) levied more than $1.4 billion in penalties.

Switzerland's UBS was given the biggest fine of $800 million, Citigroup will pay $668 million, JPMorgan must pay a $662 million penalty, Royal Bank of Scotland was fined $634 million, and HBSC $618 million.

Charges against individuals are expected.

The investigation started eighteen months ago and is continuing. Evidence shows collusion between traders began in 2009.

Barclays said it had not reached a settlement, and Germany's biggest bank, Deutsche Bank, is still under investigation. Deutsche Bank is one of the few banks to have already dismissed currency traders over probes involving alleged Forex manipulation.

The Swiss Financial Market Supervisory Authority, Finma, was the first to confirm it had uncovered illegal currency rate rigging in April. That was followed by the US, UK, Germany and a dozen other regulators across four continents who are examining the manipulation of currency at major banks in the largely unregulated foreign exchange market.

Many regulators worry currency rigging is the next Libor scandal when banks manipulated the Libor interbank lending rate by setting it low against the dollar in order to mask their financial problems. In that case over $6 billion in fines have been doled out to dozens of banks.