Traders being investigated by U.K. regulators for the suspected rigging of global interest rates are unlikely to face criminal charges while their firms may suffer record fines, people with knowledge of the probe said.

Britain's Financial Services Authority is scrutinizing evidence of attempted market abuse as well as failures in banks' systems and controls, which carry civil penalties, said the people, who declined to be identified because the inquiry is private. To file criminal charges in England, the regulator would need to show traders successfully manipulated the rate.

The FSA is among regulators looking into whether banks tried to manipulate the London interbank offered rate, the benchmark rate for $360 trillion of securities, to hide their true cost of borrowing, and whether traders colluded to rig the benchmark to profit from interest-rate derivatives.