Royal London Asset Management's David Varley is leaving his role as manager of the £429m Japan Growth fund this month, to pursue opportunities outside the industry.
Jonathan McClure will take on the running of the Japan Growth fund as RLAM seeks another permanent manager.
As a result of Varley's departure, OBSR has withdrawn the fund's A rating.
Varley joined RLAM in December 2007 as an equities fund manager.
Previously he spent six years at Old Mutual Asset Managers UK, where he was most recently deputy manager, assisting with the running of the Japanese Select fund.
He started his career as a European equity analyst with Fidelity Investments in 1999.
The Japan Growth fund returned 28% over the three years to 21 March compared to the IMA Japan sector average of 13.4%. Over one year it fell 5.4% compared to an average decline of 5.8%.
Clients to be compensated by end of 2018
Rolled out to 25 schemes next month
Mean gender pay gap now 16.64%
26 years in financial services
Some 150 claims still to process