Six men, including three leading City financiers, suspected of running an insider dealing ring were arrested during raids today.
The suspects were held as officials searched 16 homes and businesses in London, the South East and Oxfordshire.
The Financial Services Authority (FSA), which led the operation, said it was the largest of its kind to target suspected insider traders.
More than 140 of its investigators were supported by staff from the Serious Organised Crime Agency (Soca).
An FSA spokesman said the six men included two senior City professionals at well-known institutions and one hedge fund financier.
They were arrested on suspicion of being involved in a sophisticated and long-running insider dealing ring.
It is believed City workers passed inside information to traders, both directly and via middlemen, who used the sensitive data to cash in.
Suspicions were first raised in late 2007 over a series of transactions involving the six men.
Investigators were questioning the suspects and examining seized computers, mobile phones and documents.
The move was the latest chapter in a crackdown on insider trading by the City regulator.
They were the fifth set of arrests to take place since 2008 and the watchdog has secured five jail terms for financiers convicted of insider trading.
Three further cases are currently working their way through the criminal justice system.
Last month the FSA fined Mehmet Sepil, chief executive of Turkish oil firm Genel Energi, £967,005 for dealing in Heritage Oil shares using inside information.Reuse content