Gartmore's suspended fund manager Guillaume Rambourg used the Bloomberg instant messaging system to "direct" trades for nearly a year, the Sunday Times reports.
Jeff Meyer, Gartmore's chief executive, told the paper Rambourg circumvented a company rule introduced on 15 May last year.
Staff were given training sessions and asked to take a test to make sure they understood that they could no longer direct trades.
"That is what has caused us the most concern about Guillaume - it is a behavioural issue. He knew what the rules were but continued as he had before," said Meyer.
Rambourg, who owns 4% of the firm, did not appear to have profited from his actions. "I'll jump off this building if he was doing it for profit," Meyer added.
The rule banned fund managers from telling Gartmore's trading staff which brokers to use on share deals. Before it was introduced, fund managers would use the company's own IT systems to pass messages suggesting that business be directed to a certain firm.
Compliance officers discovered Rambourg had continued to make recommendations - not via the company IT system, but through the Bloomberg messaging system.
Rambourg said he would co-operate with the current internal investigation. "I accept Gartmore's reason for instigating this investigation and I am co-operating fully. I am sorry if my actions have inadvertently caused colleagues and clients uncertainty," he said in a statement, adding: "I am committed to Gartmore and to its ethos."
Square Mile’s series of informal interviews
'An entirely different beast': How have emerging markets, Asia and Japanese equities evolved over the past decade?
Talking Strategies: In the video below, Jupiter's fund managers discuss how changing tides over the past two decades have resulted in emerging markets, including EMD, and Asia and Japanese equities to become a core part of many wealth manager portfolios....
Spent 20 years with Aviva
Latest news and analysis