with complete disregard for the interests of its customers," Tracey McDermott, director of enforcement and financial crime at the FCA, said.
Lloyds TSB Bank was fined 28 million pounds in December for the way it encouraged staff to sell products that customers did not need, while Clydesdale Bank was charged 8.9 million pounds in September for a mix-up in the amount it charged more than 40,000 mortgage clients.
State Street said in a statement it deeply regretted the failings and had worked hard to enhance its controls over the past few years. The firm also dismissed staff in 2011 who were centrally involved in the overcharging.
"Their behavior was unacceptable and a significant departure from the high standards of conduct and transparency that we expect and certainly not consistent with the manner in which our employees act on behalf of clients every day," it said.
(Read more: Ax falls across Britain's banking sector)
The FCA said that when those responsible for the mark-ups were contacted by the client, the staff claimed both to the client and later to State Street U.K.'s compliance department that the charging was an inadvertent error.
They arranged for a substantial rebate to be paid but failed to disclose the existence of further mark-ups on other trades.
State Street was given a 30 percent discount on its fine for settling with the FCA at an early stage.
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