LONDON—Employees of some of the world's largest financial institutions conspired with a former bank trader to rig benchmark interest rates, British prosecutors alleged Thursday, a sign authorities have their sights on an array of banks and brokerages.

The U.K.'s Serious Fraud Office this week charged former UBS AG and Citigroup Inc. trader Tom Hayes with eight counts of "conspiring to defraud" in an alleged attempt to manipulate the London interbank offered rate, or Libor. Mr. Hayes appeared in a London court Thursday, where prosecutors for the first time detailed their allegations against him, including a list of institutions whose employees Mr. Hayes allegedly conspired with.

Mr. Hayes, who was charged with similar offenses by the U.S. last December, hasn't entered a plea to either country's charges. He wrote in a January text message to The Wall Street Journal that "this goes much much higher than me."

The charges read in court Thursday accuse Mr. Hayes of allegedly conspiring with employees of eight banks and interdealer brokerage firms, as well as with former colleagues at UBS and Citigroup. Each of the eight charges accused Mr. Hayes of "dishonestly seeking to manipulate [Libor]…with the intention that the economic interests of others would be prejudiced and/or to make personal gain for themselves or another."

The banks include New York-based J.P. Morgan Chase & Co.; Germany's Deutsche Bank AG ; British banks HSBC Holdings PLC and Royal Bank of Scotland Group PLC; and Dutch lender Rabobank Groep NV. Prosecutors alleged Mr. Hayes also worked with employees of ICAP PLC, Tullett Prebon PLC and R.P. Martin Holdings Ltd., which are London-based interdealer brokers that serve as middlemen between bank traders.