Six men were arrested Tuesday on suspicion of insider trading after a major probe by Britain's Financial Services Authority watchdog.
The FSA regulator said the men included two senior professionals at well-known City of London institutions and a hedge fund professional, following its largest operation against insider dealing.
Details of the businesses involved were not released but reports said an employee at Deutsche Bank was being investigated.
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"We are cooperating with the authorities as they look into this matter," a spokesman for the bank said.
A member of staff from the London office of hedge fund Moore Capital was also under investigation, the BBC reported. A spokesman for the US-based fund told the broadcaster that the firm was cooperating fully with the inquiry.
Officials searched 16 homes and businesses in London and the surrounding counties, seizing computers and documents.
The six were arrested "on suspicion of being involved in a sophisticated and long-running insider dealing ring," the FSA said.
"It is believed that the city professionals passed inside information to traders (either directly or via middlemen) who traded based on this information and have made significant profits as a result."
Some 143 FSA investigators were supported by staff from the Serious Organised Crime Agency (SOCA) as part of a joint investigation running from late 2007.
It is the latest move by British regulators as they step up a crackdown against insider dealing.
The FSA said Tuesday's detentions were the fifth set of arrests it had made over insider trading since 2008, and the first joint operation with SOCA.
Up until now its investigations had led to five prison sentences related to insider dealing and three other cases were working their way through Britain's justice system, said the watchdog.
