Two men and three women have been arrested following early morning raids as part of an investigation into insider dealing and market abuse.
The arrests were made after City watchdog the Financial Services Authority (FSA) and police swooped on properties in London, Lincolnshire, Leicestershire and North Yorkshire.
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The FSA confirmed the two men arrested were aged 37 and 62, alongside three women aged 39, 51 and 63.
It said the arrests were not linked to any other ongoing insider dealing probes.
It comes as part of a crackdown by the regulator, which is currently prosecuting six other individuals for insider dealing.
The FSA has so far secured 21 convictions for insider dealing and has stepped up its efforts to tackle market abuse over the past few years.
Investment banker Thomas Ammann, who worked at the London offices of Japanese bank Mizuho, was among the most recent convictions.
He was jailed for two years and eight months in December for two counts of insider dealing and two counts of encouraging insider dealing. Those convicted of insider dealing face a fine or up to seven years in prison.
Fund management firm Schroders confirmed one of its employees was among those arrested and said the staff member was immediately suspended. The group stressed the company was not involved in the investigation.
It said: "The FSA has informed us that the allegations relate entirely to this individual's personal actions. Schroders is not subject to any investigation. There is no indication of any detrimental impact on our clients or financial results."