State Street Corporation has incorrectly charged some $200m (€184m) or more to its asset servicing clients, mainly in the US, it announced yesterday.

The mistake covers an 18-year period.

The figure is based on a preliminary assessment and represents around half of a rough total of $400m that was invoiced for certain expenses during that period.

State Street will reimburse any affected clients at the end of review, including interest, and make any required improvements to its billing practices, it said. 

The actual amount to be compensated could differ materially from the preliminary figure, it added.

In its 2014 financial year, the categories of expenses under review represented less than 1% of State Street’s total asset servicing fee revenue of $5.1bn.

State Street will provide more information on the billing mistake in its fourth-quarter earnings release and call on 27 January.