"We can confirm one man was arrested in Billericay," a spokeswoman for the U.K. Serious Fraud Office said in a statement. She said officers from City of London Police helped in the operation, but declined to specify which investigation the arrest was in connection with.

Billericay is in the south eastern English county of Essex.

A City of London police spokeswoman declined to comment on the identity of the individual or whether they had been released on bail after questioning, saying only it was an SFO-led investigation.

An RBS representative could not be immediately reached for comment.

RBS said on Nov. 12 it was reviewing the conduct of over 50 current and former members of staff and had started disciplinary action against six people, three of whom were suspended pending further investigation.

Last month U.S., Swiss and British civil authorities fined six banks a total of $4.3 billion for failing to stop traders from trying to manipulate the foreign exchange market, and criminal authorities in the U.S. and U.K. have continued related investigations.

U.S. Attorney General Eric Holder also said last month he expected some resolutions of the Justice Department's investigation to come soon. He previously confirmed prosecutors had enlisted undercover cooperators as part of its multi-year probe, suggesting individuals could face charges in addition to banks.

The SFO announced it had opened its own investigation in July.

In British proceedings, arrests are usually made at the onset of an investigation, rather than as in the U.S., at the culmination of a probe.

(Reporting by Kirstin Ridley in London, writing by Aruna Viswanatha in Washington; Editing by Bernard Orr)