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By David Enrich

LONDON--British prosecutors have filed criminal fraud charges against former UBS AG (UBS) and Citigroup Inc. (C) trader Tom Hayes for allegedly trying to manipulate benchmark interest rates, in the first effort by U.K. authorities to seek criminal penalties against someone they allege was involved in manipulation of the London interbank offered rate, or Libor.

The U.K.'s Serious Fraud Office in a statement said Mr. Hayes has been charged with eight counts "of conspiracy to defraud in connection with the investigation by the Serious Fraud Office into the manipulation of Libor."

Mr. Hayes's lawyer wasn't immediately available for comment Tuesday.

Mr. Hayes and a former colleague were charged with conspiracy to commit fraud by the U.S. Justice Department last December. But a successful prosecution by the U.K. would likely ensure Mr. Hayes wouldn't face the U.S. charges, thanks to U.K. double-jeopardy laws that preclude someone charged in the U.K. from being extradited to face similar charges in another jurisdiction.

The trader hasn't publicly commented on his alleged role in Libor manipulation, beyond writing in a January text message to The Wall Street Journal that "this goes much much higher than me." His lawyer declined to comment Monday.

A 33-year-old British citizen living near London, Mr. Hayes was arrested by the Serious Fraud Office last December and then released on bail. Since then, he has been cooperating with prosecutors, who are trying to build cases against other UBS and Citigroup employees who allegedly worked with Mr. Hayes to manipulate benchmark interest rates, according to people familiar with the case.

A person familiar with the charges said they relate to Mr. Hayes's alleged actions when he worked as a yen-derivatives trader for UBS and then Citigroup, and his efforts to coordinate his alleged rate manipulation via interdealer brokers at ICAP PLC and R.P. Martin Holdings Ltd. The companies declined to comment.

Write to David Enrich at david.enrich@wsj.com

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