LONDON — Six former brokers — some with colorful nicknames, like Lord Libor and Big Nose — were “willing and enthusiastic” in helping a former trader at UBS and Citigroup manipulate an important global benchmark interest rate known as Libor, British prosecutors said on Tuesday as a criminal trial of the brokers got underway.

The trial — the second this year in Britain to focus on the Libor scandal — came after a half-decade investigation that has led to billions of dollars in fines and has rocked the reputations of some of the world’s biggest banks, including Barclays, the Royal Bank of Scotland, UBS and Deutsche Bank.

The brokers on trial worked at the British financial firms ICAP, RP Martin and Tullett Prebon Group and have been accused of conspiring to manipulate the London Interbank Offered Rate, or Libor.

Prosecutors said on Tuesday that the six men assisted Tom Hayes, a former trader at UBS and Citigroup, and others to profit by rigging Libor, which helps determine the borrowing costs for trillions of dollars in loans.