(Reuters) — The first three men to face trial in connection with a global investigation into a rate-rigging scandal that has rocked the financial industry pleaded not guilty in court on Tuesday.

Tom Hayes, a former UBS A.G. and Citigroup Inc. trader has been charged with eight counts of conspiring with staff from at least 10 major banks and brokerages to manipulate Libor benchmark interest rates between 2006 and 2010.

Terry Farr and James Gilmour, former RP Martin brokers who were arrested alongside Mr. Hayes in Britain last December and later also charged with two and one count of conspiracy to defraud, respectively, by Britain's Serious Fraud Office, also pleaded not guilty. All three are out on bail.

The pleas pose a challenge to SFO head David Green, who has staked his reputation on the success of high-profile investigations such as the sprawling investigation into the manipulation of benchmarks such as the London Interbank Offered Rate, or Libor.

U.S. and European authorities have fined 10 banks and brokerages around $6 billion to date and have charged seven men with criminal offences in connection with the rate-rigging scam. Regulators are also now investigating how other benchmarks are set, such as in foreign exchange and swaps markets.

Libor rates, designed to reflect the wholesale cost of loans, are used to help to price hundreds of trillions of dollars worth of financial products worldwide, ranging from derivatives to mortgages.

Prosecutors allege Mr. Hayes conspired to defraud with staff of UBS, Citigroup, JPMorgan Chase & Co., Royal Bank of Scotland, ICAP, Tullet Prebon, at least one employee of Deutsche Bank A.G., Rabobank and HSBC Holdings P.L.C., Messrs. Farr and Gilmour and another employee of RP Martin while he worked in Japan.

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"They dishonestly agreed to procure or make submissions of rates ... which were false or misleading in that they were intended to create an advantage to the trading position of Tom Hayes and others and deliberately disregarded the proper basis for the submission of those rates, thereby intending to prejudice the economic interests of others," the indictment alleges.

Judge Jeremy Cooke set a trial date for Mr. Hayes for January 2015. The trial of Mr. Hayes, who last December was also charged with fraud-related offences by U.S. prosecutors, could take 12 weeks, lawyers said.

The provisional trial date of Messrs. Farr and Gilmour, which is expected to take around six weeks, has been set for September 2015, in part to allow the SFO time to bring a case against further alleged co-conspirators.

Mr. Green had been hoping to charge more individuals in connection with the Libor investigation around this autumn.

UBS, which paid $1.5 billion to U.S. and European regulators last year to settle Libor-rigging charges — the biggest Libor-related fine to date — declined to comment, as did Citigroup and ICAP.

RP Martin, Deutsche Bank, Rabobank and JPMorgan also declined to comment. Tullett Prebon and HSBC did not immediately respond to requests for a comment.

Mr. Hayes joined Swiss bank UBS in Tokyo in 2006, becoming a senior trader of interest-rate derivatives indexed to yen-denominated Libor. In late 2009, he left UBS to join Citigroup in Tokyo. He left the U.S. bank less than a year later.

While Mr. Hayes was at UBS, Messrs. Farr and Gilmour are alleged to have conspired with him and other UBS employees, another RP Martin employee, an employee of Rabobank and one at HSBC, among others, by trying to influence yen-denominated Libor.

Mr. Farr is also charged with conspiring with Mr. Hayes and others while Mr. Hayes worked at Citibank.