Gartmore's Guillaume Rambourg is being investigated for favouring certain brokers when carrying out dealings, Reuters understands.
According to the report, a Gartmore source said there was no timeframe for an end to the investigation, and that no other fund managers at the group were under scrutiny.
Rambourg was suspended pending the outcome of an internal investigation into whether he had been "directing trades".
Reuters said while Gartmore officially declined to say what "directing trades" referred to, the company source said the probe had related to favouring certain brokers when carrying out dealings.
Gartmore fund managers must send their trades to a central desk for execution, City A.M. reports. It understands Rambourg's alleged breach involved trades made outside the usual central desk system - although it says it is not known what, if any, advantage he is alleged to have gained.
Reuters says a source close to the FSA confirmed the suspension of Rambourg was not connected to the joint investigation on insider dealing launched by the Financial Services Authority and the Serious Organised Crime Agency.
A source close to Gartmore said the timing of the suspension was "a completely hideous coincidence".
Gartmore's actively-traded hedge funds could generate tens of millions of dollars in commission, making them one of the top hedge fund clients in London for brokers, Reuters added.
£300bn of liabilities
View from the front row
Transfer from occupational scheme
Appointed by FCA and PSR boards